A hole on the Moon that scientists have studied from orbit for more than 15 years is about to be examined from the ground. NASA has selected a team led by the Planetary Science Institute to send instruments to the Marius Hills Pit and test whether it opens into an underground lava tube.

The investigation, called GIMLI, short for Geophysical Instruments for Marius Lunar pit Investigation, was chosen through NASA's Payloads and Research Investigations on the Surface of the Moon program, known as PRISM. PSI announced the selection on Oct. 1.

The question is more than a geological curiosity. A stable cave could shield astronauts from radiation and the Moon's extreme temperature swings, which makes the answer important for long-term lunar exploration.

A Skylight Spotted From Orbit

Japan's Kaguya orbiter spotted the pit around 2009, and NASA's Lunar Reconnaissance Orbiter has imaged it repeatedly since. NASA has described it as a possible skylight into a lava tube in an ancient volcanic region called the Marius Hills.

The pit is roughly 65 meters, or about 213 feet, wide, Tech Times reported. A lava tube forms when the outer surface of a lava flow cools into a crust while molten rock keeps moving underneath. When the lava drains away, it can leave a hollow tunnel. If part of the roof collapses, it creates a skylight like the Marius Hills Pit.

Orbital data have hinted at a void below. Tech Times reported that a 2017 study combining Kaguya radar with gravity data from NASA's GRAIL mission provided strong evidence of a large, stable void beneath the area. But orbital measurements cannot confirm how big the space is, how far it extends, or whether it is intact.

Seismic Waves Return to the Moon

GIMLI will try to "see" underground the way geologists map rock layers on Earth. Its toolkit includes active-source seismic sensors, ground-penetrating radar, a gravimeter and cameras to photograph the surface and the pit walls, according to PSI.

Active-source seismology works by deliberately creating small vibrations at the surface and recording how the waves travel through the ground. Waves move differently through solid rock than through empty space, so the pattern of returning signals can reveal hidden voids. Radar adds a second view of buried layers, and the gravimeter can detect the slightly weaker pull above an empty cavity.

The method has not been used on the Moon since the Apollo era, when astronauts set off charges and thumpers to probe the subsurface. "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," said PSI senior scientist Than Putzig, the project's principal investigator.

The last crew to run such a survey was Apollo 17 in December 1972, Tech Times reported. Those experiments revealed layering in the rock beneath the landing site, but no instrument has deliberately sent seismic waves through lunar ground since. Planned Artemis-era seismometers are designed mainly to listen passively for natural moonquakes rather than probe a specific target.

Putzig said combining the tools is what makes the plan powerful. "These methods together will allow us to get a much better understanding of subsurface properties," he said, according to ScienceDaily.

Shelter for Astronauts in the Artemis Era

Caves matter because the lunar surface is harsh. With no atmosphere or magnetic field, the Moon is exposed to cosmic radiation and solar particles, and surface temperatures swing by hundreds of degrees between day and night. Several meters of rock overhead could block much of that.

PSI said such caves "may one day provide a safe haven for astronauts to escape the radiation and extreme temperature swings present at the lunar surface." That makes them a target of interest as NASA's Artemis program works toward a sustained human presence on the Moon.

The mission is designed to produce useful science even if no cave is found. Data on the pit and surrounding rock could still explain how the pit formed. "Confirming a substantial lava tube would give us an insight into how volcanism operated on the Moon," said deputy principal investigator Gareth Morgan.

PSI director and CEO Amanda Hendrix framed the selection as a step from remote observation to direct measurement, saying Putzig's team is "taking a scientific question we've been studying from orbit" to the surface. Ground truth from GIMLI could also help scientists judge which of the other pits spotted across the Moon are worth exploring.

Partners and the Road to Launch

The team includes Honeybee Robotics, a Blue Origin company that will help develop instruments, along with Boise State University, Johns Hopkins University, the Lunar and Planetary Institute and the University of Oslo. The Norwegian Space Agency is providing the ground-penetrating radar, PSI said.

GIMLI will reach the Moon through NASA's Commercial Lunar Payload Services program, which pays private companies to deliver science instruments. It does not yet have a launch date or a confirmed lander, and PSI did not disclose the mission's cost.

Key questions remain open, from how large any cavity is to whether a lander can safely reach the rugged pit area. The bottom line: for the first time since Apollo, scientists plan to send seismic waves through lunar rock on purpose, and the answer could show whether the Moon has natural shelters waiting for future explorers.

What Readers Want to Know

What is the Marius Hills Pit?

It is a hole in an ancient volcanic region of the Moon, first spotted around 2009, that scientists think may be a skylight into an underground lava tube.

What is GIMLI?

GIMLI is a NASA-selected investigation led by the Planetary Science Institute that will use seismic sensors, radar, a gravimeter and cameras to study what lies beneath the pit.

How will the instruments detect a cave?

They will send small vibrations and radar signals into the ground and measure how they return, along with gravity readings. Empty spaces change all three signals.

Why do lunar caves matter?

A cave could protect astronauts from radiation and extreme temperature swings, making it a possible site for future shelter.

When will GIMLI launch?

No launch date has been set. It will be delivered through NASA's Commercial Lunar Payload Services program.

What if there is no cave?

The data would still help scientists understand how the pit formed and how volcanism worked on the Moon.