Perhaps you've heard in the news about the US (and perhaps other countries), trying to travel to Mars. One of the goals of the project is to finally either confirm or deny the existance of life on Mars, be it now or thousands of years ago.
Many people are complaining, as this mission is expensive and doesn't produce a material result. (It wouldn't be worth bringing back any materials that didn't have a large scientific interest, and mining is done in tons anyway.) Well, for material results, I've got something for you that I read in a magazine once.
Terraforming is a slow process that would make Mars more Earthlike, until we could build cities and wildlife reserves on it. It'd be like gaining another Eurasia for all of earthly life. (Mars is smaller than earth, and the lowest land on Mars would become flooded by the new ocean.)
The first step would be to crash Mars's two moons into the surface, as the moons aren't as scenic as ours and would be inside the Martian atmosphere by the time we finished, which would crash them anyway. Then have satellites release CFCs from the surface. This would not erode the Ozone layer as it does on earth, because Mars does not have an ozone layer. The thickening of the atmosphere would increase the temperature, which would allow for better options. At this point, we would release large amounts of methane and CO2 into the Martian atmosphere, which is useful to us as both of those are essentially waste in our atmosphere.
By this point, Mars is like the polar regions of earth: cold and miserable, but able to support some life. Lichens would be seeded at strategic points on the Martian surface. Mars has some water to sustain them, but probably not enough for, say, a penguin, or a polar bear. We would want to add more. NASA would find asteroids in the asteroid belt rich in water, and crash them into Mars. As it warms, small lakes would develop. More CO2 and methane would keep it from freezing back over.
In addition to providing oceans and life support, vaporized water is a greenhouse gas, further raising the temperatures. We're at the three quarters completed point, and now some regions of mars resemble Siberia. The wetter areas would support arboreal trees.
We continue to add CO2, but now we're planting more plants. The plants break down the CO2 with the power of the sun. The carbon becomes their food and bodies, the oxygen is released into the atmosphere. The original CFCs have probably decayed or escaped into space, so an ozone layer would form, protecting Martian life from the powerful radiation of the sun. Water would also need to be added. Contaminated earthly water could be used too, if bacteria are added to break down the pollution.
It would also be wise to find a way to reactivate the magnetic core of Mars. Earth's molten core provides a magnetic field that in addition to aiding in navigation through the use of compases, also reflects harmful radiation from space.
At the end of the project, mars has oceans, plants, and the deserts on which plants continue to expand into. We add animals now, including ourselves. Any humans there build cities, and run civilizations, be it as a colony of the sponsoring nation, or independantly. Space travel would connect the two planets economically, and I imagine the Martian population booming from the rich resources and lack of people to compete with the colonists.
All in all, this project would cost trillions of dollars and take over a thousand years to complete, and there is some risk of Mars slowly de-terraforming, returning to the lifeless husk it is today. I am absolutely convinced that it is worth doing