I for one thought this about the cork:
As Ben's teacher told him, the volcano on the Island erupted a long time ago. There is proof of this from all the dried lava. Perhaps, when it erupted, it became unstable...specifically underneath that cavern.
And to keep the pressure and energy at bay, whatever civilization living there at the time, presumably the ones who built the statue and the Temple and everything with hieroglyphics on it, they fashioned a "cork" and dug a hole in the ground and put it in there.
Perhaps the one who did this was the Woman who made Jacob protector. Perhaps the light dimmed because the volcano began to erupt, so she had to do her job but putting the "cork" in. But when she did, and the light came back on, maybe her soul was fused as well, creating the first (or maybe not the first) "Smoke Monster".
That's how she would know what happened if you went down there, and why there's a cork there. And that's why once Desmond removed the cork, all the pressure began to spill out from underneath the Island, destroying it. But once it was put back in, it held the pressure in again, and the light came back on.