An underground moon cave that stretches tens of metres beneath an open pit could be a potential lunar base for future astronauts, according to scientists.
Researchers said this is the first lunar tunnel to be discovered that could be accessible to humans.
Previous caves found on the moon do not feature any entry points, they added.
The hollowed passage lies beneath a pit about 100 metres wide in the Sea of Tranquillity, a dark region on the near side of the Moon that can be seen with the naked eye.
The “milestone discovery” comes as Nasa prepares to send its first crewed mission to the moon in more than 50 years.
Scientists have long been suspected the Sea of Tranquillity chasm – along with 200 other pits on the Moon – may have tunnels lurking beneath.
Leonardo Carrer, an assistant professor at University of Trento in Italy , said: “For the first time, we have located and accurately mapped a cave that is actually accessible from a pit on the lunar surface.
“We were able to obtain the first 3D model of a part of the cave’s actual shape.”
Lorenzo Bruzzone, a professor at the university, added: “These caves have been theorised for over 50 years, but it is the first time ever that we have demonstrated their existence.”
Underground lunar caves are ancient lava tubes created by volcanic activity billions of years ago.
Moon pits form when ceilings of lava tubes collapse.
Experts say underground caves could play a crucial role in establishing human settlements on the Moon as constructing lunar shelters from scratch requires a lot more resources and effort than colonising lava tubes.
Prof Carrer said: “Building a base on the surface of the Moon requires highly complex engineering solutions, which may be less effective than what is already provided by nature.”
For the study, published in the journal Nature Astronomy, the researchers re-analysed radar data gathered by Nasa’s Lunar Reconnaissance Orbiter in 2009.
The team developed new models of the cave beneath the Sea of Tranquillity pit, estimating it to be 30–80 metres long (98-262ft), around 45 metres wide (147ft) and 130–170 metres (436-557ft) below the surface.
Sea of Tranquillity is also where the first humans on the moon, Neil Armstrong and Buzz Aldrin, touched down in 1969.
The researchers said cave itself may be either flat or inclined by around 45 degrees.
Prof Carrer said: “The data that we have used for our discovery allows to see only the initial part of the conduit.
“We expect that it is longer than the portion that have identified and modelled.”
The moon is hostile to human life and its surface is exposed to cosmic radiation that is up to 150 times more powerful than Earth.
The lunar surface is also vulnerable to frequent meteorite impacts and extreme temperatures, ranging from 127C to minus 173C.
Previous research has suggested underground caves have an average temperature of around 17C, creating cosy conditions for astronauts.
The researchers said these caves may also give astronauts easier access to critical resources such as water ice and other minerals.
Commenting on the study, Mahesh Anand, professor of planetary science and exploration at the Open University , said: “This work confirms the presence of hollowed structures underneath the lunar surface, which are occasionally visible from outside through the collapse or breaches in their roof.
“On the moon, these structures are found inside lava flows, similar to those on Earth.
“The future exploration of the Moon through extended human presence would require protection from the harsh environment and micrometeoroid impacts.
“In that context, these underground structures could provide a suitable location for habitation purposes.”
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