I recently created a large glass dome thing at the bottom of the water pit where Rayblon's Japanese shrine is.
http://i.imgur.com/A4jpTST.jpgThe main entrance to the glass dome is the coordinates are 1611.45 x -280.11
http://i.imgur.com/7Ejl233.jpgBear in mind I built it much too large and about half of it is still flooded. I got tired of trying to unflood it block by block after awhile so I gave up half way through. Its a huge area.
Now then, with that being said, there is a small cube within the flooded area which is unflooded. In this cube is the closest shape I could get to a subway tunnel entrance. It is a uselessly long staircase down to the pits of the earth. After that it just abruptly stops. I meant to continue building but I'll admit I just got bored of it after awhile. So just so you all know there is a stair case going down to nothing just a few blocks above where you can no longer dig down.
It currently goes nowhere but fill free to make it go somewhere if you wish.
http://i.imgur.com/O1Him6j.jpg / http://i.imgur.com/kBfdRWm.jpg
Also, the 10th step on that staircase is made of glass. http://i.imgur.com/8PccMgg.jpg
There is a reason for this. remove the glass and you will find a flooded tunnel that reaches straight down to the core. Once down there, there are several winding tunnels that are not flooded but eventually open up into a large naturally formed flooded cave. http://i.imgur.com/1MpVDCz.png / http://i.imgur.com/dPwm0GM.png / http://i.imgur.com/qJWEltd.png / http://i.imgur.com/NYrdz7q.png
In that cave I have carved out a hole to the surface where it comes out on a forested cliff over looking the Japanese shrine.
http://i.imgur.com/cmY1nUK.png / http://i.imgur.com/Ne1wzUi.png
I am saying all this incase someone wishes to investigate. The still half flooded glass dome and the underground escape tunnels are up for grabs. I am bored of it and I am moving on. But as I write this they are still there. Mainly cause it was a pain in the ass to build and thus would probably be a pain in the ass to destroy. If anyone wants to claim it as their own, feel free, but remember there is still a lot of flood damage so have fun unflooding about half of it.