I found a series of translucent boxy outlines that appear to be an unfinished building on the ocean floor of a Minecraft Map Seed.

I went to checkout the structures and fell right through them. Then was unable to swim back through them to reach the surface again. I wound-up digging a trench underneath, sealing it with dirt blocks to recover my character's oxygen. Then getting back to the surface by digging a tunnel a short distance away from the boxy structures. 

The screen grabs included show the structures I'm referring to and the "chunk base" picture shows the Minecraft Bedrock Seed Number along with the coordinates to locate the structures.  

Minecraft Bedrock seed number (-5999202942967258207)

Coordinates (X:209, Z:927)