The Blue Grotto (La Grotta Azzurra) in Capri, Italy. The opening to this cave is almost under water, so you paddle in on a tiny boat, and the water is lit from below by sunlight coming in through the tiny hole.
Chile, Patagonia: " beautiful marble caves carved into passageways and caverns. These amazing marble formations were sculpted by erosion into three main marble formations: La Capilla (the Chapel), El Catedral (the Cathedral), and La Cueva (the Cave). The impressive labyrinth of marble caves are large enough for a small boat to glide into. "