I see what's happening here

I was down in my beach-front mine trying to correct the monster situation. Everywhere I went, there were tons of monsters and I couldn't figure out where they were coming from. I finally found some very dark areas up near the ceiling that I hadn't noticed before. I got them all lit up and found this:

Down at the bottom there, you can see some iron ore... but up above my head I saw a whole lot of sand. It may have taken a while, but I know the signs when I see them now. I'm supposed to go in there and pull out the iron ore which is going to cause all that sand to cave in on me. And you know what usually accompanies a sand cave-in? Water. I was almost certainly under the ocean. I figured I had two options:
1. Reinforce the ceiling so I could take out the iron
2. Stand somewhere safe and tap the sand forcing a cave-in.

Since I didn't have any good cave-in pictures, I decided on the latter. Here's the result:

Sand and water everywhere. Turns out I was under the ocean and it took a while to plug the massive hole and clear out the sand. But it got done eventually and I got my small amount of iron ore and a couple of pictures to show the process of a cave-in. Would've been better as a video though...

What to build?

 I sort of felt like my train track was a pretty epic project. It had demanded tons of my time, and I was left with something pretty amazing and functional. No longer would I have to spend an entire Minecraft day walking to my floating castle. But because it had been so great, it left me wondering what to do next. So I decided on a glass tower in the ocean. I figured it could also serve as a fast and not-confusing entrance to my most valuable mine. So I went downstairs to one of the largest chambers and started tunneling up. I figured it was under the water because of all the waterfalls. I was pretty confident I'm come up under the ocean but I wasn't worried about drowning. I was absolutely right to not worry about drowning. I didn't realize I'd come up under sand and be buried alive. Harumph.
At this point I figured it would be best to tunnel from the top down. I tried my best to figure out where my tunnel was going to come up, and this was what I decided. This tiny stone marker 10 feet off the beach.

When I was out there putting up the marker, I discovered I was a little off. There was a hole in the sand nearby... which is obviously where the sand collapsed on me. So I built a well.

And then I enlarge the well

And then I replaced the stone walls with Glass

That seemed time consuming... and the end result was the base of a tower not far from the beach. So I tried again a little farther into the ocean. This one was going to have a larger base with the water being deeper. I built the glass walls first and filled it with dirt. That was also time consuming but this is what I ended up with.

I think at this point I decided that this would be my main tower, with the other glass structure serving as an entryway of some sort. I'll get on that soon I guess.

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.