There’s an amazing geographical feature in Samoa called To Sua (Trench). It’s a giant sinkhole roughly 50 feet deep, with vertical walls, a few hundred feet from the ocean, and an underwater connection that allows the sea water (and motion of waves) to come into the sinkhole’s pool. Further, there’s a second sinkhole (called To Le Sua, I believe), which is connected to the first sink hole, but which isn’t quite deep enough to have water. I didn’t take the best photos at a macro scale, so I’m sharing this photo from Amazing Places On Earth: http://www.amazingplacesonearth.com/tosua-ocean-trench-samoa/ (my photos further down):
And I’ll attest that it’s much more beautiful in person than shown in the Chive’s recent photospread:
My photos from here on out; a shot from the water, showing the ladder into the trench
From the trench into the second (dry) sinkhole
Hanging out on the rock arch between the two sinkholes
A calculated drop from the arch into the water
And finally, a video from the bottom.