If all the water in all the oceans and lakes everywhere was gathered into a ball, see what size it would be:
Sort of brings home that fact that Earth actually has a very thin film of water that makes up our lakes and oceans.
One overly robust solar storm could blow it all away into space. Or what happened to Mars could happen to us. Mars Once Somewhat Earthlke
Or a hundred other things, equally as fatal.
It's amazing how tenuous the human existence is here on this planet. Countless natural disasters - minute on the true scale of things - could eliminate the human race forever in a blink of galactic time.
And yet we keep trying to do the job ourselves, as if the universe isn't going to accomplish the task some fine - but final - day.