A diagram of a vanishing island at low and high tide.
Vanishing island refers to any permanent island which is exposed at low tide but is submersed at high tide. Vanishing islands occur globally. There are seven vanishing islands in the Philippines and several in the San Juan Islands.
In early times, seamen were confused by this phenomenon and invented explanations for it, usually involving a massive sea monster that would let a crew land on its back before submersing itself, drowning the crew. Notable examples of these include the aspidochelone, Fastitocalon, Jasconius, Lyngbakr, Hafgufa, and various accounts of the kraken.