Caused by a big and presumably sudden geological change that sealed off the lake, Jellyfish Lake (real name Mariona Lake) is what it says on the can -- a large lake full of jellyfish.
Isolated from predators for untold jellyfish generations, they've lost the ability (and need) to sting and so the jellyfish here can be safely swum with and gently prodded. This is one of only a handful of places worldwide where you can do this -- another is Palau in the Pacific -- so it's well worth doing. The lake itself is scenic and, while the water is a little on the murky side -- visibility was perhaps five metres when we visited -- it felt overly buoyant and, once the jellyfish arrived, we stopped dwelling on what other unseen monsters may have been lurking in the darkness beneath.
A visit to the lake is generally combined with a stop at nearby Karina Beach -- a beautiful strip of radiantly white sand. There's no accommodation at Karina (at the moment) as there is some disagreement over land title, but it's a beautiful strip of sand to visit. We thought the snorkelling was mediocre, but others we talked to raved about it, so perhaps we swam in the wrong direction.