Squatter's "Rights"

We live in a world of "fiat" law (legislature), whereby stuff that does not create "conflicts" (the thing law is supposed to address), like doing certain drugs etc.,are "illegal" — i.e. punishable by force.
However there is a curious case with "squatting" amongst others, whereby if you "reside undetected" for 15yrs, you now have a legal right or claim to that land.