With all the hubbub over James Cameron’s planned dive to Challenger Deep, a little attention should be paid to the first and, at this point, only people to make it to the deepest point in the ocean: Auguste Piccard and then-Lieutenant Don Walsh. They did it in 1960 on the bathyscaphe Trieste.
The Trieste was, essentially, a dirigible. It was mainly buoyancy cambers filled with gasoline (here water:gasoline::air:helium) supporting an untethered bathysphere; there’s a huge difference in compressibility between liquids and gases, so I’ll leave the blimp vs. zeppelin distinction alone. *saunters away, whistling*
The point of this post is an interview NPR did with Capt. Don Walsh. I was blown away. Check out his Wikipedia bio – adventure scientist extraordinaire – and yet in the interview (unsurprisingly), humble and thoughtful. I’d encourage folks to give him a listen either at the NPR link or here (right click and save the mp3 locally).