No one has ever been on the moon longer than three days, largely because space suits alone can't shield astronauts from its elements: extreme temperature variation, radiation, and meteorite impacts. And while the Apollo astronauts spent 22 happy hours there, given that a moon-day last almost a month, that's nowhere near enough for an overnight visit.
For years, scientists have suspected that these lunar features were lava tubes-naturally occurring channels that form when lava turns into a hard crust. After the flow is stopped and the lava is drained, cylindrical rock formations were left behind.
The cave is about 100m wide and stretches 50km across the moon's surface according to data collected by JAXA's moon probe called the Selenological and Engineering Explorer (SELENE), Japanese news site Asahi Shimburn reports.
"It's important to know where and how big lunar lava tubes are if we're ever going to construct a lunar base", said Junichi Haruyama, a senior researcher at JAXA.
'But knowing these things is also important for basic science.
The entrance to the cave as seen from above the surface of the moon.
Jaxa recently announced that it aimed to put a Japanese astronaut on the moon for the first time by around 2030, most likely as part of an worldwide mission.
Lunar lava tubes are formed on surfaces that have a slope and may be as wide as 1,600 ft.
It found a distinctive pattern suggesting the presence of a floor and a ceiling of a lava tube.
SELENE's radar system wasn't created to detect lava tubes - it was built to study the origins of the moon and its geologic evolution.
There has always been a theory that the dark craters found on the surface of the moon are entrances to giant caves that were left behind very long ago by flowing lava.
That's good evidence, but it was all but confirmed by cross-checking the data to NASA's GRAIL mission, where the space agency was looking for areas where mass was lower than expected.
For this to happen, safe habitats such as those provided by lava tubes will be needed to protect astronauts. "They are also, potentially, large enough for quite significant human settlements-you could fit most of the historic city centre of Riga into a lunar lava tube".