This is a dome under water.
I think you all know how to build under water 🙂 either you build a room and scoop out the water with an empty bucket or you fill the room first with e.g. soil and then remove it ^^
Now please don't rip me apart or anything. It's my first idea I'm uploading and I built it over the PE version. I'm sure I could have done a lot with Redstone, but in the PE version we just got the tools, at least part of them, and I'm not that familiar with them yet.
From the inside :)
I wanted to light it up extra much since monsters spawned there all the time. But no chance, they keep coming. All the time...
I have built a tunnel so that you can reach the dome well :)
I hope you like my idea/ the elaboration :)