In the face of intense solar radiation and blinding red dust storms, the first humans who travel to Mars will need a protective habitation module.

After all, it's not like they can just live in a cave.

Or could they?

A handful of proponents say caves created long ago by cooling lava would be the cheapest, largest and most protective places to expand human outposts.

Mars was once volcanic, scientists say, and it shares features with Earth, which is still volcanic.

Among the common features are myriad types of caves left behind as lava flowed across the landscape and then cooled.

Wind Cave near Bend, Oregon was created by lava flows.

Caves will not likely be a first home, in part because they have yet to be mapped. The first explorers would need a place to live while they search for lava tubes and modify them to be livable. But proponents say caves could potentially provide a vast network of rooms and hallways where a colony could expand activities and store supplies.

"Depending on where we wish to establish our outposts [other considerations being mineral resources, access to permafrost as a source of water] martian lava tubes would offer substantial volumes already shielded from cosmic rays, solar flares and the sun's untempered ultraviolet," says Peter Kokh, a past board member of the National Space Society.

Kokh said cave entrances might be strewn with boulders along steep slopes.

"Attractive as they are, there are some practical problems with gaining access that may mean that we don't do this until we have a more capable operation on Mars," Kokh said in an e-mail interview. "But the tubes are a definite asset and it would be stupid not to plan on making the most of their existence."

Kokh said similar caves on the moon could also support human colonies. (He also muses that lunar caves would be the most logical place to look for possible calling cards left long ago by other intelligent beings.)

R.D. "Gus" Frederick, an explorer of dark places in the Pacific Northwest, is a fitting person to extol the benefits of martian cave dwelling.

By trade, Frederick is an instructional technologist at the Oregon Public Education Network, but his passion is clearly caves. He has been prowling in them since he was a kid.

Skylight Cave near Sisters, Oregon is a lava-flow cave with a hole in the top that lets the sun shine in.

Frederick has explored lava tubes in Oregon, Washington and elsewhere. He first brought up living in martian lava-tubes in 1996, and he presented the idea at the annual Mars Society convention last year. He describes one way the potential human homes form:

Like wax from a candle, flowing lava freezes in place. A crust forms over the top, insulating the liquid underneath, which continues flowing.

When the source of magma is exhausted, the remaining liquid lava drains out, leaving the crust and a hollow interior. Frederick says some terrestrial caves resemble subways, with ceilings more than 20 feet (6.1 meters) tall.

Where the crust is thin, a portion sometimes collapses and creates a skylight. Such holes, Frederick says, not only provide a way into a cave but could also serve as a place through which to direct sunlight.

Frederick says photos of Mars show lava-flow landforms that are similar to those on Earth, indicating the possibility of caves, which he says are probably larger than earthly caves.

Skylights on martian terrain indicate possible caves below.

"The obvious disadvantage is that you cannot choose where to put your lava-tube cave, like you can with a traditional surface habitat," Frederick says. "So any initial habitats will most likely be on the surface."

NASA and others have mocked up various overgrown canisters intended to serve this purpose.

But caves don't need to be hauled Mars-ward, proponents point out. And caves could provide protection against the harsh radiation there, which occurs because the thin atmosphere barely filters the sun. Some researchers have speculated that martian caves might hold stores of water ice.

Frederick goes further than some proponents of cave dwelling by suggesting that a translucent, inflatable balloon could be used to seal openings while still allowing light in. Settlers might then fill the cave with oxygen. Add a little water and an entire ecosystem might be possible.

"In some way it could be seen as a very primal experience," Frederick says. "Leaving the womb of Mother Earth to live within Mars. And perhaps help Mars become another Earth."