A stunning new video shows the adventures of an international team of astronauts who have been exploring the inner spaces of deep caverns in Italy. It's an exercise in looking for life on other planets, which will most likely be underground.

The search for life on other worlds goes back centuries, and despite tremendous improvements in telescopes, along with a fleet of robots that have visited every planet in the solar system, we have yet to find any little green men, signs of lost civilizations, fossils, microbes or even a single piece of alien DNA.  

Nothing. Nada.  

Maybe that's because we have been looking for life in all the wrong places.

The centrepiece of that search for someone else out there has been Mars because it is the most Earth-like planet, with icy polar caps, seasons and a landscape of canyons and sand dunes reminiscent of Earthly deserts. It was once thought to be the home of an advanced civilization that built huge canals that criss-crossed the planet, bringing precious water to desert cities. And if they could engineer canals, they could build spaceships and send them on invasions to the blue, water-covered Earth.

Now that we've been there, we do see signs of water moving across the surface, but they were made naturally by rivers, not clever Martian engineers. Sadly, those rivers, lakes and oceans only existed about 3 billion years ago, when Mars was a warmer, wetter world, a place where life could have evolved as it did on Earth. Since then, the oceans of Mars have dried up and liquid water became permafrost as the planet entered a permanent ice age. The atmosphere has thinned out, turning the whole world into a cold, dry desert where unfiltered ultraviolet light from the sun would eventually kill anything trying to survive on the surface today. So if Martian life did appear then, it doesn't appear to be on the surface now.

Gale Crater on Mars

This composite image looking toward the higher regions of Mount Sharp on Mars was taken on September 9, 2015, by NASA's Curiosity rover. (NASA/JPL-Caltech/MSSS)

But who is to say life could not thrive underground?

On Earth, life forms have been found in caves and within the rock walls of our deepest mines. Thriving without sunlight, these extremophiles live on underground water and minerals from the rock. In fact, it has been estimated that there could be as much life in the Earth as there is on it.

So perhaps we should be looking for life in Mars, which will require spelunkers in space.

The European Space Agency's CAVES, (Cooperative Adventure for Valuing and Exercising human behavioural Skills) program takes six astronauts from Europe, the U.S., Russia, China, and Japan deep underground for two weeks at a time to practise techniques for looking for life, as well as surviving as a team in the subterranean environment. Special tools are used for collecting samples as well as climbing gear for navigating narrow passages. Working in total darkness, using harnesses and ropes to get around, it is almost like taking a spacewalk.

Caves on Mars?

Mars has the potential for underground caverns, although none have been found yet. The largest volcanoes in the solar system tower above the red plains, one of them, Olympus Mons is about three times the height of Mt. Everest. Volcanoes are a sign of heat within the planet, which could melt permafrost and possibly provide warm, wet habitats in underground caverns. Volcanoes on Mars may also have lava tubes, such as those found in the volcanoes of the Hawaiian Islands.

If life is found underground on Mars, there will be ethical issues around contamination, both from Earth and from Mars back to Earth. Many science-fiction movies have portrayed the horrors of alien invaders wreaking havoc on helpless humans, and we could just as easily infect Martian organisms with our bacteria. That story has been repeated more than once in the history of exploration.

The fact that life can exist in so many extreme environments on Earth — from caves, to scalding water, to ice at the south pole — raises the hope that we will find life in the extreme environments of other planets such as Mars, or ice-covered moons of the giant planets.

We may just have to dig deep to find it.