Are we alone?

The universe is enormous, so the chances of us being the only living creatures are small. Although we think we are intelligent and that we know a lot about Space, we have only explored a very small area. We might be the only creatures that can travel in Space, but it is unlikely.

In fact, some people say that we might have been visited by aliens. These people point to 'wonders' such as Stonehenge in Britain and the Nazca lines in Peru as proof that aliens have been here.
So, what are the chances that there is life out there?