The bathyscaphe Trieste reached the bottom of the deepest location in the earth's crust - The Mariana Trench. This feat has always amazed me as well as their discovery of life at that depth. At over 35,000 feet deep, the pressure at the bottom of the trench is about 8 tons per square inch. Unreal. Wiki article
Watched the documentary about this the other day. Before i saw it i was under the impression that people had never been down that deep. It's truly amazing, especially considering the technology with which it was achieved