Everyone who has gazed at Mars with the dream of settling there has always pondered the idea of terraforming the world. That is to say, people have dreamt of turning Mars into a second Earth, albeit smaller and cooler, so that people can walk the surface of the planet without spacesuits and live like we do here on Earth. There have been many science fiction stories written on the subject and, even though I don't care for the books, amongst the most popular of them is Kim Stanley Robinson's Mars trilogy.
More recently, Elon Musk and other people have suggested terraforming Mars as well. Elon's suggest of nuking the polar ice caps might be a touch...extreme. It makes sense from the POV of giving humanity a second home and also from the POV of all that extra room. Mars has the same surface area as the land area of the Earth, more or less, and even with oceans covering 60% of the surface after terraforming, that's a lot of new land: greater than 3 and a half times the land area of Asia. It would never be as balmy as most of Earth: Mars gets less than half the sunlight Earth does.
A terraformed Mars would be a new world free of worries whether or not someone else is getting squished when you settle the land: there are no natives. Call a terraformed Mars the the New World Guilt Free.
Reality, however, has a way of interfering with one's dreams: there has been some recent results that have been...less than encouraging.
There has long been a question as to what happened to the Martian Atmosphere. Was it completely lost? Was it sequestered in the rocks of Mars? We know it existed, because there was at one time a northern ocean and rivers, but it is gone now. It seems the answer has been found. The atmosphere was simply lost.
In fact, what is left on the planet, sequestered and still in the air, is only enough to raise the atmospheric pressure to 7% of Earth sea level. That's roughly the same as being at 61,000 feet (~18,000 meters). This means any terraforming will require a lot of volatiles (water, carbon dioxide, nitrogen, etc) to be imported. And the amount is...enormous. Roughly as much as is in the whole of Pluto. Plus or minus.
So, today, my friends, a dream took a hit from reality. It might not be dead, but it definitely is beyond our lifetimes.