Something doesn't seem right here...

uhh hello water nice to see you aren't going anywhere.

 

Its not water, some purple tarish-jelly-thing At least, that was my conclusion.

 

This is a bug I think. I had this happen in the first beta version of the game.

What happen is basically the planet generates the liquid FIRST, then the dungeon/structure, but the liquid doesn't register there's now air underneath it and so it doesn't flow down.
Similiar to fine sand and gravel. Disturb it and avalanche.

 

@Andre it's definately water, I could swim in it. haha. And this is the newest version so I posted this for the lols and to warn Chucklefish. haha

 

It was there in the first version as well.