There is a winding network of underground caves on the Moon, according to evidence gathered by an international team of scientists, including one extending from the base of a pit near where Apollo 11 landed on the lunar surface.
The team made the discovery by analysing data collected by the the NASA Lunar Reconnaissance Orbiter in 2010.
They found evidence of a subsurface cave on the Moon extending over 200 feet from the base of a pit.
This pit is 203 miles northeast of the Apollo 11 landing site, Mare Tranquillitatis (Sea of Tranquility), where Neil Armstrong and Buzz Aldrin became the first humans to set foot on another body beyond Earth.
Scientists say the cave could stretch for miles beneath the surface of the lunar mare.
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Discovering the Moon caves
Just like on Earth, there are subsurface caves on the Moon, and scientists have suspected this for decades.
Evidence of these lunar caves was gathered by lunar orbiters even before the Apollo era.
A pit was then confirmed in 2009 using images captured by the Japan Aerospace Exploration Agency's Kaguya orbiter.
Many more Moon caves have been discovered across the lunar surface by images and heat measurements taken by the Lunar Reconnaissance Orbiter.
Pits discovered by the Lunar Reconnaissance Orbiter range in size from 5 metres to over 900 metres.
"These caves have been theorized for over 50 years, but it is the first time ever that we have demonstrated their existence," says Lorenzo Bruzzone, professor at the University of Trento, who took part in the study.
"In 2010, as part of the ongoing LRO NASA mission, the Miniature Radio-Frequency (Mini-RF) instrument acquired data that included a pit in Mare Tranquilitatis.
"Years later we have reanalysed these data with complex signal processing techniques we have recently developed, and have discovered radar reflections from the area of the pit that are best explained by an underground cave conduit.
"This discovery provides the first direct evidence of an accessible lava tube under the surface of the Moon."
Lava tubes
What has formed this network of caves below the Moon's surface?
Scientists suspect they could be formed like 'lava tubes' on Earth, whereby molten lava flowed beneath a surface of cooled lava in the Moon's early history.
Or a crust may have formed over a river of lava to produce a long, hollow tunnel.
If the ceiling of one of these lunar lava tubes collapses, it exposes a pit that can be discovered by a lunar orbiter, pointing to an extended subsurface cave.
Moon caves for future astronauts
As humanity begins sending probes back to the Moon and NASA prepares for its Artemis III mission to return human feet to the lunar surface for the first time since the Apollo missions, there is much discussion on how to ensure the safety of future astronauts.
One solution could be the use of underground caves as a way to protect lunar explorers from the dangers posed by living and working on the Moon.
“Pits would be useful in a support role for human activity on the lunar surface,” says Robert Wagner of Arizona State University, Tempe, Arizona.
"A habitat placed in a pit — ideally several dozen meters back under an overhang — would provide a very safe location for astronauts: no radiation, no micrometeorites, possibly very little dust, and no wild day-night temperature swings."
