this is not a journal title

Okay, someone explain to me, why is the oil gushing out of the well? I picture the oil as being inside an underground lake, with the well being like a straw going down to it. But when you put a straw into a glass of juice, the juice doesn't suddenly start gushing up the straw, it sits inside the glass until you start sucking on the straw. In the case of a juice box, the juice does start gushing out of the juice box, but that's because you're squeezing on the package while holding it to jam in the straw - is there something "squeezing" on the oil in the ground? Or is it heat causing the pressure, and if so then why hasn't all the oil leaked out and stabilized that way?