Thanks for all of the interesting responses. I wound up reading a lot of the materials on this diving history sub forum, along with random walks through some of the Wikipedia links and other Scubaboard posts. (Including some of the deep air records stories -- and the sad stories of some of the failed attempts.) Really fascinating stuff -- there seems to be never-ending debate over a huge range of issues for those trying to go deeper (gas mixes, decompression algorithms, equipment, etc.).
I'm happy with my shallow quarries for now (and maybe some shallow Great Lakes wrecks next summer). A few years down the road, maybe I'll work up to the U-352 dive in North Carolina, but that's probably as deep as I will ever see. The diving and mountain climbing stories sort of make you realize the narrow range of environmental conditions required for our bodies to operate properly. However, people will always want to see how far, how fast, how deep, how high, etc. that they can go (e.g., parachuting from 120,000 feet is pretty nuts, too).
I'll probably watch some more of those old Cousteau shows with my son. Still good stuff to watch. After all of these years, glad I finally decided to actually start diving.