A Canadian astronaut who has spent his life training to travel to the stars is heading in the opposite direction, venturing deep underground on an expedition meant to simulate a space mission.

David Saint-Jacques is the first Canadian to participate in the European Space Agency's Co-operative Adventure for Valuing and Exercising human behaviour -- also known as CAVES.

Saint-Jacques is on a team along with NASA's Mike Fincke, the European Space Agency’s Andreas Mogensen, Russia's Nikolai Tikhonov and Japan's Soichi Noguchi.

The team spent a week training for the expedition in Sardinia, an Italian island in the Mediterranean, then ventured underground Friday, entering into a labyrinth of caves that stretches tens of kilometres underground -- much of it unexplored and unmapped.

"Being in a new environment, unusual circumstances, having to work together -- the focus is really how we work together despite the stressors of the environment, the novelty and the task," Saint-Jacques told CTVNews.ca.

The CAVES project, which began last year, is meant to simulate the challenges faced by astronauts working in space, dealing with challenging scientific objectives amid cramped quarters in a foreign, isolated environment, while working with people from different cultures.

But the team won't be locked in a sterile, underground space capsule simulator. Instead they will be donning helmets and headlamps, and scrambling through the complicated, largely unmapped cave system, absent of daylight for six days.

Their goal will be to navigate and map as much of the system as they can, collect samples, conduct scientific experiments and search for life -- all while using protocols similar to those used on spacewalks.

"Nobody has systematically looked for life in these caves," ESA astronaut trainer and CAVES course designer Loredana Bessone said in a statement. "Finding life big and small is always good, so I am very excited we will be looking for exotic bacteria and cave dwellers such as anthropods."

During their time underground the team can request just one delivery of supplies or equipment -- everything else they must carry with them.

They will also test out a new communication system that they will use to stay in contact with a mission control team based near the entrance to the cave. Two briefings will be held per day as they would during a spaceflight.

Saint-Jacques said he is expecting a challenging but fascinating experience.

"I'm acutely aware that there is some danger in what we're doing, so safety is the first priority. You have to be very careful," he said by phone from Sardinia.

"And second, I know it's important to be extremely careful with human interaction on the international team. There's an extra level of awareness you have to keep of other people's reactions, just because they're from a different culture and context."

Saint-Jacques, who is from the Montreal area, is a medical doctor and engineer, and holds a PhD in astrophysics.

Though he has not been to space, he was sent by the Canadian Space Agency in 2009 to join NASA's 20th astronaut class, and last year participated in NEEMO 15 (NASA Extreme Environment Mission Operations) -- the underwater equivalent to CAVES.

"NEEMO and CAVES are very similar projects in ambition. Of course practically they're very different," Saint-Jacques said.

"NEEMO was underwater in a tiny habitat while this is in a cave, but psychologically there are a lot of similarities -- six international people living in close quarters with a lot of intimacy. We are in an extreme and completely alien environment with some level of danger, and we rely heavily on technology."

While three of the participants have experienced space travel, three -- including Saint-Jacques -- have not. He said he hopes to glean knowledge and experience that will one day serve him well on a space mission.

"It's a rare opportunity to perform a mission with an international group and learn in practice how international groups behave in these circumstances," Saint-Jacques said.

He added: "And also it's a great chance to see these beautiful caves."