Underwater Civilization

Imagine discovering an advanced underwater civilization. What does it look like and how does it function?

This provokes thoughts about extreme environments, adaptability, and unique societal structures. How do these underwater people cope with pressure, see in the murky depths, or communicate? Is their society similar to ours or is it altogether different? What innovations and adaptations are necessary for survival underwater? This emphasizes on out-of-the-box thinking, inventiveness and alternate societal functioning.