"AM: Really? During our debate, Jim Kasting said he thought Mars could be terraformed for plants, after about 40,000 years of carbon burial, but he couldn't foresee Mars being suitable for humans until millions of years later.

JBL: I have a faster plan. The idea is you dig a big hole, maybe at a 45-degree inclination, in the Hellas crater, which is already at low altitude. Maybe you need a hole of 45 kilometers, which is not so easy. But the whole atmosphere of Mars is going to go down into the hole.

Two things would happen. First of all, the pressure at the bottom of the hole would be about 1 bar - much higher than at the surface. It would also be a much warmer temperature, because when you go down, the pressure increases and the temperature also goes up. You would have enough CO2 and enough water possibly to have a greenhouse effect and liquid water.

Salty water is liquid at several degrees lower than 0 degrees Celsius, depending on the amount of salt. In fact, salty water is much less demanding than pure water in terms of the amount of greenhouse warming needed for it to stay liquid. And life likes salty water.

Possibly, such a situation might have happened in the past on Mars, without requiring a 45 kilometer-deep hole But the combination of salt water and greenhouse warming may be needed in the future for us to sustain any life there.

AM: So you foresee a habitat for plants and people rather than an attempt to terraform the whole planet.

JBL: Yes, that is absolutely right. It would be local, but after that perhaps you can start the other project. You plant trees there, and they may produce carbon, and that would be a good starting point"

http://www.spacedaily.com/news/marsexpress-05a.html