Astronauts could set up lunar bases inside underground Moon caves, scientists believe.

A tunnel has been spotted underneath the Sea of Tranquillity by Nasa satellites orbiting the Moon and analysis suggests it could be more than 250ft long.

Radar images indicate the site, which is at the bottom of the Mare Tranquillitatis Pit, the deepest known crater on the Moon, show an opening about 150ft wide.

Researchers said this is the first lunar tunnel to be discovered that could be accessible to humans and previous Moon caves did not feature any entry points.

The "milestone discovery" comes as Nasa prepares to send its first crewed mission to the Moon in more than 50 years and experts are working on how to make it hospitable for long-term human habitation.

Scientists have long suspected the Sea of Tranquillity chasm - along with 200 other pits on the Moon - may have tunnels lurking beneath.

Dr Leonardo Carrer, an assistant professor at the 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."

It is thought that there could be a system of tunnels under the lunar surface which were created by the collapse of ancient lava flows.

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 Sea of Tranquillity is the ancient lava plain where the Apollo 11 astronauts Neil Armstrong and Buzz Aldrin first set foot on the moon in 1969.

The researchers said the cave may be either flat or inclined by around 45 degrees.

Prof Carrer said: "The data that we have used for our discovery allows us to see only the initial part of the conduit.

"We expect that it is longer than the portion that has been 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 on Earth.

The lunar surface is also vulnerable to frequent meteorite impacts and extreme temperatures, ranging from 127C (260F) to -173C (-280F).

Previous research has suggested underground caves have an average temperature of about 17C (62F) 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."