It would seem NASA weren't exaggerating when they said they were going to announce a “significant find” on Mars today. They've photographed strong evidence of liquid water on Mars!
It appears to be coming from an underground source of some kind, and flowing out onto the surface. Due to the thin atmosphere and cold temperatures it quickly evaporates or freezes, but it looks to have been in a liquid state at some point.
As liquid water is considered one of the requirements for life, it once again raises questions about the possibility for microbial life on Mars.
It's funny, I was jokingly thinking to myself “OMG! Aliens!” when I read the press release this morning, and that's turned out to not be entirely inaccurate. *laugh*
The full story is on NASA's site.