The Ikka Carbonate Columns, Ikka Fjord, Greenland - the coolest underwater geological structures ever. They form from underwater seeps of Carbonate to form a mineral called Ikaite, which form these sweet pinnacles. But they are disappearing fast, most likely as a result of ocean acidification. The pinnacles are essentially dissolving as the surrounding ocean becomes more neutral, so withing a few decades these will be gone.
Definitely on my list of top 5 places to dive.