I was sailing around what I think may be the beginning of the ocean about 8,500 blocks from my spawn. I saw thru the water some light coming from the bottom of the sea... what could it be?
I decided to jump overboard and check it out... turns out it must've been a cave with a pool of lava that had some vents of seawater spilling in from above, causing the floor to be made entirely of obsidian. I'm not sure where or why there was still light showing (maybe a bug?) but it was pretty cool.
That's pretty interesting. Underwater caves have been showing up quite a bit. I even managed to escape from some creepers underground by going up a water stream that as it turned out was from the river by my home.
Definitely... in my original save, which I've been playing since 1.2 or so, I was sailing around and near the beach where I found my first NPC village, nearby I found what looked like a mineshaft entrance right at beach level, and then down through the water what looked like another. Pretty wild stuff... there's so much to explore even in the immediate area around where you spawn, and to think it just keeps going, and going, and going... it's pretty mindblowing.