If we assume that the bear cages at the hydra station were for the polar bears, how the hell did they get to the main island. I know polar bears can swim but could they really travel to another island with the tide and everything. I mean look how hard it was for Michael to get off the island.
Well, these bears are probably super smart. After all, they figured out how to get to the food faster than Sawyer could.
I suppose, rather than swim, the smart bears commandeered a boat off of the dock and figured out how to sail themselves to the other island.
Maybe that's why Desmond's boat was of importance to Ben. The Others still hadn't recovered the boat that was stolen by the bears!
I think the REAL question is how did the polar bears get on any ISLAND in the first place. I'm guessing that the Hydra Island is where the scientist observed the Island and it's inhabitants. Notice how the bull horns etc are not hidden on the Hydra Island. I also believed there to be an underwater hatch since the first lady that boone tried to save, the shark and the unknown cable leading to the ocean. No one thought that was wierd? I'm thinking that Ben's "there's no escape" is a ruse and that there is a tunnel that leads to the mainland.