Another day, another new cave system

My railway now reaches all the way back to the first house I built in Minecraft. I was over there building a railway station and filling in holes in the beach (holes I had created acquiring sand for windows). While I was doing that I bassed by a piece of sand underneath my front steps that had been bothering me for a while. I took it out and was extremely surprised to discover a new cave right in front of my house!

Obviously I went down to explore. I was running low on rail tracks so I was going to need more iron ore anyway. I was in the cave system for a very short while before I came across a bunch of monsters. I couldn't quite get at them so I started digging a narrow path to come at them from the side. It seemed like the reasonable thing to do... but no. Cave-in!

Unlike the previous cave-in, this one was serious. I was completely buried and couldn't escape. I died and when I re-spawned my first thought was for all the equipment my corpse had left behind. I quickly ran back to see what could be salvaged and saw a complete mess of sand and ocean. After several minutes I managed to stop the flow of ocean water and had recovered my gear:

While I was excavating, I heard monster noises. A lot of them.

Because of the recent cave-in I was very careful about where I was going to dig and I eventually managed to find a room filled with monsters of every kind. After dying a few times I managed to clear them out and discovered the most expansive cave system I've found so far. There are several enormous caverns:

And they're sometimes separated by very narrow openings. I barely explored down here at all because I was carrying a lot of stuff that I didn't want to lose. I tried finding my way back to the entrance but that proved extremely difficult. A few times I thought of just digging straight up and out, but I was worried I was still under the ocean somewhere. Eventually I found my way out and went to finish my railway.