I'd wanted to make something underwater for a while - glass being an obvious material - plus lava (if possible).

Doing anything in water is a pain - so I filled a hole in the 'sea':

http://gettingwood.blogspot.com/2011/11/undwerwater-structure-project-part-1.html

...then built my circles of lava in glass etc.:

http://gettingwood.blogspot.com/2011/11/undwerwater-structure-project-part-2.html

..and finally recreated still water over the structure (and glass encased rail track):

http://gettingwood.blogspot.com/2011/11/undwerwater-structure-project-part-3.html

- maybe nothing on some of the incredible structures gone before on servers etc., but built as a relative noob, alone on normal/survival mode with ALMOST no cheating (I flew a little bit - making the 'lake' towards end : / - it's hard freaking work and when you're that close but so far... well...)