Even though the moon is relatively accessible and the only other celestial body where humans have walked, it's a fairly inhospitable place. With no atmosphere and no magnetic field, it's pummeled by extreme temperature variation, radiation, and meteorite impacts, all factors that could threaten the long-term (or short-term) survival of any prospective lunar colony.

The good news is, scientists have identified a shelter on the moon that might just protect us from these threats: an immense lava tube that's big enough to house a city at least the size of Philadelphia, reports Phys.org.

This sublunar cavern, found in the Marius Hills region of the moon, could be the ideal place for astronauts-turned-cavemen to hide from the hazardous elements on the surface. It was located by researchers at JAXA, the Japanese space agency, using radar data from the SELENE spacecraft to look for echo patterns that might reveal open entrances to giant underground chambers. The size of the tube is difficult to fathom; data suggests it is at least several kilometers in length and at least one kilometer in height and width.

"It's important to know where and how big lunar lava tubes are if we're ever going to construct a lunar base," said Junichi Haruyama, a senior researcher at JAXA. "But knowing these things is also important for basic science. We might get new types of rock samples, heat flow data and lunar quake observation data."

Establishing a lunar colony could be a huge boon to the prospects of further space exploration. The lower gravity on the moon means that launching spacecraft from its surface would take far less fuel, drastically reducing the cost of a launch. A lunar base would also make for the ideal hub between asteroid mining operations and Earth, not to mention a pretty spectacular tourist destination.

The discovery comes at a time when the Trump administration has announced plans to redirect America's focus in space to the moon.

"We will return NASA astronauts to the moon – not only to leave behind footprints and flags, but to build the foundation we need to send Americans to Mars and beyond," claimed Vice President Mike Pence at a recent meeting of the National Space Council.

The Marius Hills lava tube could very well be the site of this new age of lunar colonization.