Earth's Balance

While sitting in AP Environmental Class, observing the many elements that contribute to our Earth's balance, I began to ponder... How does Earth stay so balanced? Why is it that water, that makes up 3/4 of the planet, does not simply crush the land? We seem so steady and balanced, and yet we are so weak and vulnerable. We always say that there will be an apocalypse; an end to everything we know. But in addition to this, we say "this will not happen for years." But how many years have we been saying this? How long exactly DO we have until we are done? If the cavemen were saying/drawing "this will not happen for years," then are WE in the years they were referring to? Are WE the apocalypse? Have we been put on this earth to destroy, to deplete, to eliminate? And if we were, can we change our purpose?