There's always talk on populating the moon or populating Mars and I feel like "We barely look after our own planet given how much mining and cutting we do, wouldn't it be best to just leave these planets alone?"
Especially given Mars does not have a natural atmosphere for us, so we'll probably be quick to pollute it than make it hospitable for us.
Classic human.
Running away from problems is easier than solving it.
But what if Mars turned out already populated?
It's just we couldn't see it with our eyes?
Some kind of astral or divine beings.
Then when we try to populate there, they'll rebel against us.