I've got a headcanon that waaaay back in the day newly-throned Bonnie was a big reader, and found a rucksack containing a bunch of books that were written by some human chap named Simon Petrikov. Upon finding out that this man was not only still alive but also that her friend was pretty much his daughter, she practically begged Marceline to go see him. Marceline was reluctant obviously but felt like visiting her old friend because what harm could it be?
As you can see, it... didn't end well.
Now Marceline's running off in a hissyfit with some doll she stole, Bonnie's pissed because she can't keep up, and there is a huge storm dominating the previously green mountainside.... hold on, this snow doesn't seem natural - crazy magic is afoot here. What's happening to the mountains?
And... the gems?
Basically another big headcanon is that the early Ice King was one volatile sunofabich. The Ice Kingdom used to be really freaking dangerous.