Before we settle Mars, we must return to the caves. That's the advice of astronomers who have found a cavern in the Moon capable of holding a city the size of several Sydneys.
Space is dangerous.
Beyond the protection of Earth's magnetic field, radiation from the Sun and the stars can tear a deadly path through any astronaut's DNA, said news.com.au.
And we've got a lot to learn about how to protect them from it.
It's one of the greatest challenges we face in sending - and keeping - astronauts on Mars.
Which is why our international space agencies and corporations are shifting their sights to the Moon.
It's seen as both a stepping stone and a sounding board for a future Mars mission.
And the ideal site for humanity's first populated offworld colony may have just been found.
It also has the potential to accelerate humanity's path to Mars.
Engineering a surface lunar base poses many challenges.
People inside need to be shielded from the bombardment of charged particles, wild variations in surface temperature (up to 300C between day and night), as well as from micrometer strikes.
So what if you didn't have to worry about such things?
A study published this week in the science journal Geophysical Research Letters has confirmed that an enormous, open lava tube has been found in the Marius Hills region of the Moon.
It could offer a ready-made shielded site within which human structures can be built.
Lava tubes are nothing new. They are naturally occurring caverns formed when channelled lava begins to thicken around the edges, forming a tube.
Sometimes, when the lava drains, it can leave behind it a hollow cavern.
This has happened on Earth (there's one in northern Queensland).
We've now found one on the Moon.
But it's much bigger than anything we've found down here.
"It's important to know where and how big lunar lava tubes are if we're ever going to construct a lunar base," says Japanese space agency (JAXA) researcher Junichi Haruyama.
And the initial results indicate it's big. Real big.
The tubes showed up on a radar scan. But they also showed up on a gravitational analysis.
For it to be detectable in gravity data, the tubes would have to be several kilometres in length - and at least one kilometre in height and width.
That's big enough to hold many of Earth's largest cities.
If it actually resists.
It will need to be mapped and scanned by a future robotic probe mission to accurately assess its size and condition.
And the hunt is already on to find similar lava tubes on Mars.
JAXA used radar data from the Japanese lunar orbiter SELENE and NASA's GRAIL gravitational field mission to seek lava tubes laying beneath the lunar surface.
They struck paydirt near the previously discovered Marius Hills Skylight - a collapse leading to an underground cavern.
The radar pattern from the surrounding terrain was distinctive, showing what is likely to be reflections from both the ceiling and floor of several tubes scattered around the hole.
The SELENE radar was not designed to look far underground.
But GRAIL could.
"They knew about the skylight in the Marius Hills, but they didn't have any idea how far that underground cavity might have gone," says Professor Jay Melosh of the GRAIL project. "Our group at Purdue used the gravity data over that area to infer that the opening was part of a larger system. By using this complimentary technique of radar, they were able to figure out how deep and high the cavities are."
inding such an enormous protect space beneath the surface of the Moon is suddenly very relevant.
As a recent International Astronautical Congress in Adelaide demonstrated, the world's space agencies have refined plans for a collective effort to reach Mars.
First, we must build a 'village' on the Moon.
It's also an objective the United States' revived National Space Council agrees with.
"We will return NASA astronauts to the moon - not only to leave behind footprints and flags, but to build the foundation we need to send Americans to Mars and beyond," Vice President Mike Pence recently announced at its first meeting in decades.
WHY THE MOON?
It's an ideal testing ground. And a road-stop.
We really have little idea how well our technology will perform in raw, deep space.
So it's safer to find out at a testing ground just days away from help - the Moon - instead of at least a year away - Mars.
But getting to Mars may also benefit from having facilities on and around the Moon.
Assembling and fuelling a ship in space is easier than attempting to launch all of it from the surface of Earth.
And the Moon may offer an easy source of fuel - if enough water ice can be found to be harvested.
This could be catapulted into space due to the Moon's low gravity, instead of needing heavy-lift rockets to carry such a volatile cargo off Earth.
Also, we already have the technology.
We've been to the Moon before. Some 48 years ago, in fact.
We can do it now.
The six-month to one-year travel time to Mars still presents a multitude of unsolved problems.
And there's generally only a two-week period every two-years where the planets align for a suitable 'shot' can be made.
We can go to the Moon anytime.
And what we learn on the Moon will help propel us to Mars - and ensure we actually survive long enough to get there.