NASA wants to find out whether a hole on the Moon leads into a cave — and whether that cave could serve astronauts as a ready-made shelter. The project, GIMLI (Geophysical Instruments for Marius Lunar pit Investigation), was selected on 30 September under NASA's PRISM program, which sends research instruments to the lunar surface. It is led by the Planetary Science Institute (PSI), and no launch date has been set.

The target is the Marius Hills Pit, a vertical hole in a region with a complex volcanic history, discovered by Japan's Kaguya orbiter. Like other lunar pits, it may open into a lava tube — a long void left when lava drained away beneath an already solidified crust. Radar data from Kaguya offered a hint in 2017: an echo pattern near the pit that the authors of a Geophysical Research Letters study read as a possible lava tube.

To confirm a void, the team plans three surface techniques. Ground-penetrating radar supplied by the Norwegian Space Agency will send radio waves underground and infer the subsurface structure from the reflections. Researchers will also generate controlled vibrations and measure how they travel through rock — cavities change the signal, and per project lead Nathaniel Putzig, active seismics have barely been used since the Apollo landings. A gravimeter will look for minute differences in gravity that can betray a hollow, with cameras on top. Honeybee Robotics, which PSI describes as a Blue Origin company, is building the seismic system, gravimeter and cameras and will operate them on the Moon. The payload is to be delivered by a commercial lander and rover under NASA's CLPS program.

If the pit does lead into a tunnel, the payoff would be strategic: a lava tube would shield people and equipment from radiation, micrometeorites and extreme temperature swings without anyone having to build a bunker. Even a negative result has value — the pit's walls expose rock layers that are otherwise hidden, letting scientists reconstruct how lava once flowed and how often eruptions occurred.

NASA notes one limitation itself. The Marius Hills Pit sits on the near side of the Moon, near the equator, while the planned lunar base is in the south polar region — so the pit is not a candidate site for the first settlement, but it is a test case for whether lunar voids are habitable at all. GIMLI is one of three PRISM projects intended to prepare astronauts for surviving on the Moon, alongside LEMS-SP, a measurement station at the south pole, and DISCO, which aims to make the first direct measurements of ice in permanently shadowed depressions.

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