The second of my underwater base maps. I responded to the suggestions given on these forums by making this one much bigger; it's actually as big as the Source Engine would let me make it. However there is one source limitation that I simply cannot overcome:
THERE CANNOT BE A SURFACE!! If I make an underwater base with glass windows, there simply cannot be an above-water island or "elevator" like so many people suggested. I really tried to make one, but the main problem was that the env_fog_controller entities are server-side, not client-side. I can't try to explain this, but just trust me.
There is coral, kelp, little coral fish (just in the base), and a sunken ship. I also threw in some underwater theme music to listen to as you explore the ocean floor.
Finally, I built an airlock to easily launch your submarine contraptions.
Here's the download link: