But how did it get there?
There’s a big tunnel on the Moon, and we can’t be 100% sure how it got there – but it could be a handy place for humans to live, provided there are no aliens inside.
The underground cave has been found in the Sea of Tranquility, the 500-mile wide basin where Neil Armstrong and Buzz Aldrin became the first astronauts to walk on the Moon almost 55 years ago to the day.
For almost as long, scientists have suspected there are tunnels on the Moon, but had never discovered one with an opening, until now.
It was found thanks to Nasa’s Lunar Reconnaissance Orbiter, which surveyed the Moon and allowed scientists from the University of Trento in Italy to create a 3D model of the cave.
Thought to be up to 80 metres long, 45 metres wide and between 130 and 170 metres below the surface, it stretches out beneath a pit around 100 metres wide in the Sea of Tranquility – a dark region on the near side of the Moon which can be seen with the naked eye.
But the big question is not what it looks like, but where did it come from?
Leonardo Carrer, a researcher at the university, said: ‘The most likely explanation for our observations is an empty lava tube.’
On Earth, lava tubes are the caves left behind by underground lava flows – earlier this year, scientists discovered evidence of humans living in one 7,000 years ago.
Lava tubes on the Moon are thought to have formed the same way and, in the future, could help humans colonise it.
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 -173C to 127C.
But 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, and would be cheaper to make habitable.
Leonardo Carrer, an assistant professor at University of Trento in Italy, 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.’
However, it might require a few stairs building in, as the team isn’t sure whether the cave is flat, or at a 45-degree angle.
It was discovered thanks to old data gathered by the orbiter’s Miniature Radio-Frequency (Mini-RF) instrument in 2010 that the team reanalysed.
The discovery comes as Nasa prepares to send its first crew to the Moon in more than half a century.
Four missions are planned so far for the Artemis programme. The first, Artemis I, successfully tested Nasa’s new Orion spacecraft and space launch system, which flew on a 1.4 million-mile journey to the Moon and back.
Artemis II will repeat the feat, but with astronauts on board, while Artemis III will land astronauts back on the lunar surface.
However, both have been delayed a year, to 2025 and 2026 respectively, as both the spacesuits and landing craft are still in development.
Elon Musk’s SpaceX has been hired to develop the vehicle that will transport astronauts from the Orion spacecraft to the Moon itself, but is still being tested.
