Natural tunnels left by lava flows under the Moon’s surface may reach gargantuan proportions of up to 5 kilometers, a new model predicted. They are a promising location for future manned lunar bases.

Lava tunnels are formed when the external layer of a lava flow cools down enough to form a hard shell, while the hotter inner part remains fluid. The lava probably drains leaving a natural tunnel behind. Such formations are abundant in some regions on Earth, and are suspected to be present on the Moon as well.

Thanks to low gravity, the lunar tubes may be broader than their counterparts on Earth, and a new computer-modeling project by David Blair and colleagues from Purdue University in West Lafayette says the theoretical threshold is 5 kilometers.

"We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the moon," Blair said, at the 46th Lunar and Planetary Science Conference (LPSC) in Texas.

"This wouldn't be possible on Earth, but gravity is much lower on the moon and lunar rock doesn't have to withstand the same weathering and erosion. In theory, huge lava tubes – big enough to easily house a city – could be structurally sound on the moon."

Illustrating his theory was an image of his home city of Philadelphia sitting inside a massive cave. More realistic expectations would be around 1 kilometer, but it is still bigger than the mere hundreds of meters promised by earlier estimates. The previous attempt to model lunar lava tubes was published in 1969.

The researchers calculated the stability of lava tubes according to their size, roof thickness, rigidity of the cooled lava and whether the cooling left a single layer of lava or many thin layers.

Among the many difficulties builders of a manned lunar base would face is protecting it from a hostile environment, which includes extreme temperature ranges, exposure to cosmic radiation and risk of micrometeorite impact. Siting the base inside a lava tube would make the task much easier.

As an extra benefit is that the tubes are formed along mild slopes and would be found in areas where the Moon’s plains border highland regions, so a base placed in one would have good access to a variety of terrain and corresponding resources.

A lunar lava tube is yet to be discovered, although there are suspected entrance points left by collapsed walls.