The surface of the moon is hostile. A newly found cave could be a lifesaver.

The first astronauts since the Apollo era will land on the lunar surface later this decade. If NASA’s Artemis program proceeds as planned, it will bit-by-bit, establish a permanent presence on the moon, around the water-rich south pole.

That’s no easy feat. The moon is one of the most extreme and hostile environments in the solar system: It features wild temperature swings in and out of sunlight, it’s occasionally rocked by intense moonquakes, and it’s almost always soaked in galactic and stellar radiation raining down from above.

“The lunar surface is hostile to humans and machines,” says Tracy Gregg, a planetary volcanologist at the University at Buffalo.

Although artificial structures raised on the lunar surface will provide shelter, it would be helpful if the moon itself offered some natural defenses. A study published this month in Nature Astronomy provides the first direct evidence of the existence of such natural shelters. By looking through old radar data taken by a probe orbiting the moon, researchers found that a suspicious looking pit close to the Apollo 11 touchdown site isn’t merely a pit, but a lengthy cave—a volcanic tunnel forged by an ancient lava flow.

Lava tubes and caves exist on Earth, and they have provided refuge to voyaging humans from intense weather for almost 10,000 years. The cave discovered on the moon is likely to be very similar to Earth’s own, and scientists suspect it’s hardly alone: Myriad such conduits are thought to exist all over the moon.

The primordial volcanic walls of such caves will give geologists windows into the moon’s distant past, and they may also contain invaluable water-ice—which can be converted into rocket fuel.

But most importantly, if this off-world cave network exists, it could offer protection from the moon’s more otherworldly threats, from solar radiation to falling micrometeorites, says Leonardo Carrer, a researcher at the University of Trento in Italy and an author on the new study.

Hunting for lava caves

Today, the moon is a quiet, silvery desert. But it was once a hyperactive volcanic paradise, where molten rock fountained into space and rained back down as glassy tears, and where enormous seas of lava churned away. With much of its internal heat now lost, the moon’s volcanism has become extinct, leaving behind all sorts of gnarly features on the surface. (Read up on the history of lunar exploration.)

Among those features are lava tubes, hollowed-out tunnels that once insulated and channeled rivers of incandescent rock. They exist, and are still being created, on Earth today. Cold, crystallized lava tubes, caves and conduits have long been thought to hide across the moon, particularly within the maria—dark patches of frozen-over magmatic seas. 

Flying high above the surface, NASA’s Lunar Reconnaissance Orbiter’s (LRO) camera can only see pits, which may not necessarily lead to extensive caves. LRO, however, is equipped with a small radar system, which, if beamed at these pits at just the right angle, may reach their potentially lengthier depths.

Carrer and their colleagues went through preexisting radar data and zeroed in on a pit in the Sea of Tranquility, the solidified lava sea that Apollo 11 landed on back in 1969. That data revealed that the pit had open walls 430 to 560 feet below the surface skylight, leading into at least one deep cave that snakes through the lunar crust.

Scientifically speaking, exploring a conduit like this would uncover a bounty of treasures. “The caves are a unique environment that preserves the history of the moon,” says Lorenzo Bruzzone, a researcher at the University of Trento and an author of the new study. All sorts of secrets, from the history of volcanism on the moon to the composition of the moon’s enigmatic interior, may be revealed by spelunking into one. 

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The newly described lava tube is the first subterranean cave conduit ever found on the moon, shoring up the notion that there are countless more yet to be discovered. “There are probably hundreds to thousands of caves on the moon in the form of drained lava tubes,” says Gregg. And one day, they may save an astronaut’s life.

A subsurface sanctuary

Shelter is the top priority. “It's about having a ready-made habitat where astronauts can spend extended periods on the moon without contracting cancer,” says Paul Byrne, a planetary scientist at the Washington University in St. Louis.

And shelter becomes especially important in solar storms. Earth’s prominent magnetic field and thick atmosphere shield the surface from all but the sun’s most terrifying outbursts. But the moon lacks both, meaning its surface gets bombarded by that radiation whenever it’s in the sun’s firing line.

Unprotected astronauts could face radiation doses ranging from dangerous to deadly. “A solar storm could literally kill people on the lunar surface,” says Gregg. “Computers don’t like solar radiation either.”

Radiation gets absorbed and then reemitted by the lunar surface, which could also harm astronauts in the long run. “Being on the surface generally, even when the sun isn't throwing crap at us, is still not a great idea,” says Byrne. (Read more about how space impacts the human body.) 

These caves could also provide sanctuary from other threats. The lunar surface temperature can rise and fall by hundreds of degrees with remarkable speed, depending on whether it’s illuminated by sunlight. Fortunately, it’s likely that “the interior of lunar caves maintain a stable temperature,” says Carrer, offering another bonus.

Tiny meteors the size of a toaster or smaller pose no threat to Earth, because our world’s atmosphere incinerates them. But the moon has no such gassy shield. “The surface of the moon is continually being bombarded by micrometeorites, which ultimately degrades anything that’s left outside,” says Gregg.

Lava caves, then, would make for sturdy storage spaces to supplement lunar outposts being built by astronauts and robots over time. “Ready-made sub-surface habitats or storage areas would save a great deal of work for folks,” says Gregg.

South pole spelunking

These caves aren’t silver bullets for lunar adventurers. Accessing a cave hundreds of feet below the surface may require astronauts to carefully rappel down into them—not ideal if they need to quickly flee from a solar storm. 

Another issue is that the caves may not all be structurally sound. “The caves we’ve seen so far are visible because the roof has collapsed,” says Gregg. “I think roof stability is going to be a big issue.”

The moon also experiences moonquakes, which are infrequent but occasionally strong, and can last for tens of minutes. Not only might these imperil lunar outposts, but they could destabilize lava tubes. “I’d want to see the cave walls reinforced if an outpost is being build there,” says Thomas Watters, a planetary scientist at the National Air and Space Museum in Washington, D.C. “Without reinforcement, the cave walls and ceiling could collapse.” 

Crucially, space agencies are targeting the resource-rich lunar south pole for the first long-term human stations on the moon. That means they may not find plentiful lava conduits, which are more likely to be found in those maria on the side of the moon facing Earth.

But just knowing that such caves exist, after years of theorizing, still provides a measure of comfort to anyone seeking out answers to the mysteries of the moon, an orb whose fate is so closely linked to Earth’s. “What it is important is that now we have identified an accessible cave that could be the target of a future robotic mission,” says Bruzzone. “We are ready to be surprised.”