Prehistoric volcanoes used to erupt on the moon, and what they left behind billions of years ago could turn into a lunar shelter for the future.

Future astronauts face an onslaught of potential hazards once they land on the lunar surface. There is no magnetic field to protect whatever (or whoever) is on the surface, and space suits just don’t have the power to shield a moonwalker from the extreme temperature highs and lows our satellite experiences, not to mention meteorite impacts and killer radiation. Imagine your weather forecast being UV showers with a chance of meteorites. No wonder no one ever hung around there more than three days.

There is only one place to go if you don’t want to end up burned or smashed. Underground.

The newest discovery on the Marius Hills region of the moon is kind of like a built-in lunar hotel. When the moon was still in the throes of teenage angst, it was bursting with volcanoes that oozed lava. As lava flows, the upper layer hardens into a crust over the hot liquid still gushing inside. Lava tubes happen when this tunnel of fire drains and cools into a hollow void that’s better than SPF 1000 sunblock or meteorite-resistant space armor. It isn’t some uncomfortable crawl space, either. This lava tube is the size of Philadelphia.

“Lunar lava tubes are important from various science perspectives and provide potential sites for future lunar base construction,” said JAXA (the Japanese space agency) senior researcher Junichi Haruyama and colleagues in a study recently published in Geophysical Research Letters. “Since the insides of lava tubes are shielded from meteorite bombardment, cosmic radiation, or particle implantation, they are expected to be in pristine condition.”

Skylights on the moon.

Lava tubes on the moon had been speculated about but never proven to exist. JAXA was able to unearth this find—which began as an opening or “skylight”—by analyzing radar data from the SELENE spacecraft. SELENE was really designed to find evidence of lunar origins and evolution, but its radar reflections gave away an echo near the entrance and another further down. JAXA scientists reached out to scientists from NASA’s GRAIL mission, a continuing effort to collect data on the moon’s nearly nonexistent gravitational field. GRAIL was able to further zero in the suspected lava tube and distinguish where there was less subterranean mass. That meant there was a void beneath the surface.

“They knew about the skylight in the Marius Hills, but they didn’t have any idea how far that underground cavity might have gone,” said GRAIL co-investigator Jay Melosh, a distinguished professor of earth, atmospheric and planetary sciences at Purdue University. “Our group at Purdue used the gravity data over that area to infer that the opening was part of a larger system. By using this complimentary technique of radar, they were able to figure out how deep and high the cavities are.”

The automatic lunar shelter is one of many lunar advantages that have convinced the recently re-convened National Space Council to prioritize blasting astronauts off to the moon before any other destination (yes, even Mars).

The Red Planet may just have to wait.

(via Purdue University)