Oh goddamn it. I went to throw a bone or something into lava and out of my inventory to make room for the shitton of obsidian I found, thumb slipped, hit O... I was holding my diamond pickaxe.
EDIT: I just spent the last few hours exploring the MASSIVE cave system I found (it's connected to my house, actually). I finally reached a huge underground ravine with waterfalls and lava flows. I decided I'd head back the way I came, hope I didn't get lost, and lay signs along the way. Figured it would be about an hour to get back. Just as I was leaving, I noticed a waterfall that opened up into the seabed. I used the water elevator and BOOM, surfaced in a cove across the lake from my house. I built a huge pillar with torched on it to mark the spot, and plan on making it into a tube so the water can't get in and with a ladder. Can't get over how many cave systems connect in perfect ways and how many cool opportunities it opens up.