So I if just start "maintaining" some land, in about 15 years I can claim it is mine?
It's harder than it sounds.
I think you the owner has to know you are taking care of it, and does nothing about it. So, in my dad's example, the neighbor set his fence on what is now my dad's property when the old owner had it still. He then ran cattle on it for something like 20 yrs, all the while maintaining the ground and the fence. When dad when to reclaim it, the guy quickly won the Adverse Possession suit.