The passage behind the time Vault leads to an incredibly cold chamber playing host to a massive wheel. The wheel can hardly be turned by human strength alone and even if it can be moved, the person doing so is transported from the island and into another time altogether.
The wheel itself is a larger version of a donkey wheel. A donkey wheel, as most know, was used mainly in mills. Donkeys or horses would be tied to the wheel and forced to turn it to keep the mill going.
Now the frozen donkey wheel has too many risks. Donkeys and horses most certainly wouldn't be able to turn it. They would die from the cold temps. Another reason is that if a human was used to turn the wheel, though they are obviously strong enough, they would be banished from the island and who knows where they would end up.
My theory is that Dharma captured polar bears and trained them to turn the wheel. They put them in cages with mind games to test their intelligence. Then when the island needed to be moved, polar bears would be perfectly capable of turning the wheel and withstanding the freezing temperatures. There was no risk of anybody being transported in time, except the polar bears who were expendable. My guess is that when the island was moved previously, a polar bear turning the wheel was transported to Tunisia in the past. Later, it's bear bones are discovered, complete with a dharma collar around its neck
