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Natural tunnels known as lava tubes could safely house permanent bases on the Moon, scientists have said.

The underground volcanic structures have previously been proposed as ideal sites for human settlements.

Scientists have now assessed how stable these features might be, and found that tubes of 1km in size and bigger would be structurally sound.

They could protect against the challenges posed by the lunar environment.

Details of the work were presented at the 46th Lunar and Planetary Science Conference (LPSC) in Texas.

Unlike Earth, the Moon lacks a thick atmosphere and magnetic field to protect it against cosmic radiation. The absence of an atmospheric buffer also means that the Moon's surface receives more frequent meteorite impacts and more extremes of temperature.

For example, the Moon's surface temperature can vary by several hundred degrees C during the course of a lunar day.

Cave opening
Cave entrances like this one in Mare Tranquilitatis may open into lava tubes
But housing bases underground, inside lava tubes, could offer shielding against these risks.

The lunar tunnels are expected to be larger than those already discovered on our planet, because of the Moon's lower gravity. No-one has yet definitively discovered an example on the Moon, but spacecraft have revealed cave entrances called skylights that may open into lava tubes.

Using existing data, David Blair and colleagues from Purdue University in West Lafayette used computer modelling to determine the stability of lunar lava tubes with different widths, roof shapes and roof thicknesses.

"It's the first modern re-assessment of how stable these can be," he told BBC News.

He found that tunnels with widths of 1km and greater could remain standing beneath the lunar surface.

Lunar lava tubes should be stable up to about 5km wide
In his poster at the meeting, he displayed a diagram showing his home town of Philadelphia sitting neatly within a lava tube with a width of 5km - which was found to be the rough theoretical maximum for stability.

The underground structures are thought to have been formed during basaltic lava flows during the Moon's history. When the upper layer of a lava flow cools and hardens, molten rock may continue to flow beneath it. Once this drains, it can leave behind a hollow space.

Long, snaking features on the Moon called sinuous rilles are thought by some to be lava tubes that have collapsed in on themselves. However, this interpretation is controversial among scientists.

Nonetheless, researchers are continuing to look for clear evidence of their existence in spacecraft data.