For years, trivia buffs thought the world's largest island-in-a-lake-on-an-island-in-a-lake-on-an-island was a tiny island in a crater lake on Volcano Island in Lake Taal on the Philippine island of Luzon. [it is now some island in Canada, btw]
Taal Lake is also home to one of the world's rarest sea snakes, Hydrophis semperi. This particular species is only one of two "true" sea snake species that are known to live entirely in freshwater (the other is Laticauda crockeri from the Solomon Islands). Bull sharks, Carcharhinus leucas, used to be part of the lake's once-diverse ecosystem, but they were exterminated by the locals in the 1930s.