For the first time since Apollo astronauts detonated explosives on the Moon in 1972, a mission will intentionally generate seismic waves on the lunar surface — and the target is a hole in the ground that scientists have long suspected opens into one of the solar system’s most consequential voids.

NASA announced on October 1, 2026, the selection of three new PRISM scientific payload suites through its Payloads and Research Investigations on the Surface of the Moon program. The standout of the three is GIMLI — the Geophysical Investigation for Mapping Lunar Interior — which will be deployed at the Marius Hills Pit, a roughly 65-meter-wide (213-foot-wide) hole on the lunar surface that may be a skylight opening onto a vast underground lava tube. If it does, that tube could one day shelter astronauts from the Moon’s brutal radiation, temperature swings exceeding 250°C (482°F) between day and night, and micrometeoroid impacts — providing a natural ready-made habitat that no constructed shield has ever matched.

What makes GIMLI historically unusual is its instrument suite. It will combine ground-penetrating radar, an active-source seismic system, a gravimeter, and cameras into the most direct surface-based investigation of any lunar pit ever attempted. The active-source seismic component is especially significant: there has been no intentional generation of seismic waves on the lunar surface since the Apollo 17 mission in December 1972, when astronauts set out explosive charges as part of the Apollo active seismic data records.

“It’s long been a desire of mine to reintroduce intentional active-source seismic methods to planetary science, as it has essentially not been done since the Apollo astronauts conducted the first seismic surveys on the Moon,” said Dr. Nathaniel Putzig, GIMLI’s principal investigator and a senior scientist and associate director at the Planetary Science Institute (PSI) in Tucson. “Combining that method with ground-penetrating radar and gravity measurements makes it all the more exciting, as these methods together will allow us to get a much better understanding of subsurface properties — including the anticipated detection of a lava tube extending away from the Marius Hills pit.”

What GIMLI’s Instruments Will Do — and Why the Gap Matters

Understanding why GIMLI is newsworthy requires understanding what the three core geophysical methods do and why their combination matters for detecting a void.

Ground-penetrating radar (GPR) works by transmitting electromagnetic pulses into ground — typically in the microwave UHF/VHF range — and measuring reflections from subsurface features. Air-filled voids create a sharp dielectric contrast with surrounding rock, producing a distinctive reflection pattern. An orbital version of this instrument — the Apollo Lunar Sounder Experiment (ALSE) — flew around the Moon on Apollo 17 in 1972, and NASA’s Perseverance rover has since used a GPR called RIMFAX on Mars, with RIMFAX scanned Martian subsurface layers to more than 15 meters (50 feet). GIMLI’s surface-deployed GPR, contributed by the Norwegian Space Agency according to the PSI announcement, would operate at far closer range and higher resolution than any orbital equivalent.

Active-source seismic works differently. Rather than waiting for natural moonquakes to provide a signal, a mission generates seismic waves with a known source and measures how fast those waves propagate through different materials. Solid basalt carries seismic energy at one speed; regolith, another; an air-filled void, another still. The technique is standard in terrestrial resource exploration, but the Apollo missions were the only time it has been applied on another world — and only by astronauts who could deploy thumper devices and fire rocket-propelled grenades into the lunar surface. GIMLI’s mission will need to achieve this in a fully robotic deployment. The PSI announcement describes hardware built by Honeybee Robotics, a company Blue Origin acquired Honeybee Robotics in 2022, that will generate and record these waves.

A gravimeter rounds out the suite by detecting minute variations in local gravitational pull. A large underground void contains less mass than solid rock — so a gravimeter traversing the pit’s rim would register a measurable gravitational anomaly from underground voids. This approach mirrors how NASA’s GRAIL mission used orbital gravity mapping to identify a candidate lava tube at Marius Hills a decade ago: the mission recorded a gravity deficit consistent with a hollow space. GIMLI’s surface gravimeter can make much finer-grained local measurements than any orbiter.

Cameras round out the payload. Marius Hills Pit’s exposed walls already reveal ancient lava-flow layers otherwise buried under regolith — a cross-section through the Moon’s volcanic history that orbital imagery can suggest but never directly read at high resolution.

Dr. Gareth Morgan, a senior scientist at PSI and GIMLI’s deputy principal investigator, described the broader scientific value: “Confirming a substantial lava tube would give us an insight into how volcanism operated on the Moon. Lava tubes are a common feature of basaltic volcanism on Earth, so identifying them on the Moon means we could use knowledge of such terrestrial caves to better understand lunar history.”

What Are Lunar Lava Tubes, and Why Do They Produce Voids This Large?

Lava tubes form when the outer surface of a flowing basaltic lava stream cools and hardens into a crust while molten rock continues moving beneath. When the eruption ends and the lava supply stops, the liquid drains out, leaving a hollow tunnel. On Earth, lava tubes in Hawaii and the Canary Islands can reach a few tens of meters in width. On the Moon, the same process played out under conditions of dramatically lower gravity — roughly one-sixth of Earth’s — and with the high-volume flood basalt eruptions that built the lunar maria. The result, according to multiple studies using GRAIL gravity data, could be tubes more than 1 kilometer (0.6 miles) wide and tall, stable enough to remain intact for billions of years. The theoretical upper bound is staggering: calculations suggest lunar lava tube cross-sections could exceed 1 billion cubic meters (35 billion cubic feet) of total enclosed volume while remaining within the structural stability limit of the basalt roof.

The Marius Hills region is particularly promising because it is one of the Moon’s most volcanically complex areas, dense with volcanic domes and rilles. The pit itself — spotted first by Japan’s Kaguya orbiter around 2009, then repeatedly imaged by NASA’s Lunar Reconnaissance Orbiter — sits atop ancient lava flows. The 2017 Kaguya and GRAIL study combining Kaguya radar and GRAIL gravity data provided the strongest orbital evidence yet that a large, stable void extends beneath this pit — though the precise extent and geometry remain unconfirmed without surface investigation. GIMLI would be the first attempt to answer that question from the surface directly.

The practical stakes are substantial. The Moon’s surface offers astronauts almost no protection from solar and galactic cosmic radiation, is subject to continuous micrometeoroid bombardment, and swings between temperatures of approximately −173°C (−279°F) at lunar night and 127°C (261°F) in full sunlight — a range no surface habitat has to fully span inside a lava tube. A sufficient lava tube could provide all of this protection for free, without requiring radiation shielding to be launched from Earth.

“Such a cave may one day provide a safe haven for astronauts to escape the radiation and extreme temperature swings present at the Moon’s surface radiation and temperature environment,” NASA stated alongside the selection announcement.

How Does an Underground Cave on the Moon Change the Math for Living There?

The engineering math for a Moon Base changes significantly if a large verified lava tube is available. Current planning involves either constructing pressurized surface habitats with built-in radiation shielding — which requires launching large masses of shielding material from Earth — or covering habitats with regolith, which requires earthmoving equipment and time. A pre-existing tube of sufficient scale would make both approaches secondary options.

PSI Director and CEO Amanda Hendrix framed the mission’s significance this way: “Than and his team are taking a scientific question we’ve been studying from orbit and have developed a way to investigate it directly on the Moon.”

Even a negative result would be scientifically useful. If GIMLI’s instruments show that the Marius Hills Pit is simply a shallow collapse feature without a deeper structure, that rules out one of the most promising lava tube candidates and tells mission planners where not to build. The mission’s cameras would still capture the geological layering of the pit walls, and the seismic and gravity data would characterize the near-surface structure of the Marius Hills region.

GIMLI’s Place in a Three-Mission Science Push

GIMLI is one of three new PRISM payload suites announced simultaneously as part of NASA’s Moon Base program.

The Distribution of Ice and Surface Characteristics Observatory (DISCO), led by Dr. Ariel Deutsch of NASA’s Ames Research Center in California’s Silicon Valley, will search for water ice in micro-cold traps near the lunar South Pole — small permanently shadowed patches where temperatures remain low enough for ice to survive. DISCO will also study how rocket-plume interactions disturb the lunar surface, a practical concern if spacecraft are going to land regularly near a future base.

The Lunar Environment Monitoring Station–South Pole (LEMS-SP), led by Dr. Mehdi Benna of the University of Maryland, Baltimore County, will position an autonomous instrument near the South Pole to monitor seismic activity, micrometeoroid impacts, and volatile compounds over a long duration — building the baseline data on surface hazards that engineers will need before permanent hardware goes in.

Together, the three missions address the most immediate practical questions for human surface operations: Where is the water? Is natural shelter available underground? What conditions will hardware and crews face?

“NASA Science is building the ultimate interplanetary survival guide to ensure that science goes first to the lunar surface to provide our future astronaut crews with the vital information, resources, and safety precautions needed ahead of time to survive the night on the Moon,” said Nicky Fox, NASA’s associate administrator for the Science Mission Directorate.

Brad Bailey, director of NASA’s Exploration Science Strategy Integration Office, called the selections an expansion of the frontier of lunar exploration.

How GIMLI Gets to the Moon — and When

All three PRISM payload suites will be delivered to the lunar surface through NASA Commercial Lunar Payload Services, under which the agency purchases delivery as a commercial service rather than building and operating its own landers. GIMLI does not yet have a confirmed launch date — CLPS scheduling is finalized closer to flight — but with the PRISM-3 selection now complete, hardware development and mission planning can formally begin. The PRISM Step-2 deadline February 2026 accepted Step-2 proposals through February 27, 2026, and today’s announcement marks the end of the review process.

Honeybee Robotics, whose hardware heritage includes the TRIDENT drill flown on the PRIME-1 mission in March 2025 and the sample-caching drill on NASA’s Perseverance rover, will build much of GIMLI’s instrument hardware according to the PSI announcement, including the seismic system, gravimeter, and cameras.

PSI is partnering with co-investigators from Boise State University, Johns Hopkins University, the Lunar and Planetary Institute, the University of Oslo, and the Norwegian Space Agency — an international team whose radar expertise, the PSI announcement notes, will be essential to interpreting what GIMLI finds.

Can GIMLI Definitively Confirm a Lunar Cave?

GIMLI’s instruments are designed to detect whether a void exists and estimate its geometry — but the mission has limits. Ground-penetrating radar is constrained by its operating frequency: higher-frequency signals give better resolution but shallower penetration, while lower frequencies can reach deeper but with less detail. The same tradeoff governs all subsurface radar investigations, from Earth utilities inspection to Mars ice-layer mapping. The depth and extent of a potential Marius Hills lava tube — estimates from orbital data range from tens of meters to more than 100 meters (330 feet) below the pit floor — will determine whether GIMLI’s radar can fully characterize what it finds.

Active-source seismic will face its own constraints: without astronauts to deploy explosives and without the benefit of a network of geophones spread across a wide baseline, GIMLI’s seismic system will make high-priority local measurements around the pit rather than the kilometer-scale surveys Apollo achieved. But even a local seismic profile of the Marius Hills Pit would be the most detailed subsurface measurement of that feature ever made.

The gravimeter provides a complementary check: a large void produces a measurable gravitational anomaly that neither radar nor seismic can fake. Together, the three active methods and the cameras give a triangulated picture that no single instrument could provide.

If GIMLI confirms a significant void, it will shift the engineering question from “does a natural habitat exist?” to “how do we get inside and use it?” That is a different conversation — and a much more tractable one — than the one NASA has been having from orbit for the past decade.

Frequently Asked Questions

How does GIMLI plan to detect an underground cave without drilling?

GIMLI uses three complementary non-invasive methods. Transmitting electromagnetic pulses into ground lets the radar read reflections from density contrasts — an air-filled void creates a much stronger reflection than solid rock. An active-source seismic system generates seismic waves and measures how they travel through different materials; voids change both wave speed and character in detectable ways. And a gravimeter detects tiny variations in gravitational pull: a large hollow space contains less mass than the surrounding rock, producing a measurable dip in local gravity. Cameras add visual data from the pit walls. None of these methods requires digging or drilling, and together they provide cross-checking lines of evidence that a single instrument alone could not.

Why hasn’t anyone done active-source seismic on the Moon since 1972?

Active-source seismic on the Moon has required deploying an energy source — thumpers, mortar-fired grenades, or buried explosive charges — in exactly the right configuration relative to receivers. During Apollo, this meant astronauts physically placing the hardware. In the robotic era, no subsequent mission has included a dedicated active seismic capability. Passive seismic monitoring (measuring naturally occurring moonquakes) has continued on Artemis III’s planned LEMS instrument, but intentionally generating waves is technically more demanding to do without a crew. GIMLI would be the first robotic mission to close that 50-year gap, using a Honeybee Robotics-built system designed for autonomous deployment. The history of how Apollo astronauts deployed seismic explosives explains the technical challenge GIMLI must replicate robotically.

Could a lunar lava tube really shelter an entire Moon Base?

Possibly — if it is large enough and structurally sound. Kaguya and GRAIL orbital data together suggest a candidate tube at Marius Hills with dimensions that could exceed 1 kilometer (0.6 miles) in width and height. A tube of that scale would provide natural protection from radiation, maintain far more stable temperatures than the open surface, and provide a structural floor for habitats and equipment. The critical caveats are that orbital estimates of tube geometry are unconfirmed, ceiling stability over vast spans has not been independently verified from the surface, and entry would require engineered access through the existing pit opening. GIMLI’s job is to answer the first question — does the void exist at the suspected scale — before any of the others become relevant.

What happens if GIMLI finds nothing at Marius Hills?

A negative result would be scientifically and practically valuable. It would rule out the Marius Hills Pit as a lava tube entrance — focusing future habitat planning on other pit candidates or alternative designs — and provide the most detailed geophysical characterization of the region ever gathered from the surface. The seismic data would shed light on near-surface regolith properties, bedrock depth, and the geological structure of one of the Moon’s most volcanically complex regions, regardless of whether a cave exists. NASA and PSI GIMLI announcement describes the mission as trying to answer the question definitively, in either direction.