If they went to the bottom, and drilled a kilometer into the earths crust, then I'd be worried. You don't want to open up a volcanic fisher. I imagine the immense ocean pressure would dampen the explosion, and you wouldn't get a large tsunami. You might trigger some small earthquakes around the world, but I don't think it would be catastrophic. Thanks for the comment. I'll following, and i'll give you a few upvotes on your blog, when my voting power recharges.