The Mad Monkey was a legendary sunken ship with a valuable monkey shaped figurehead.

Guybrush Threepwood went in search of it in Monkey Island 2 in order to retrieve the Monkey Figurehead.

It is unknown why it sunk but the author of Great Shipwrecks of Our Century knew where it was.
Mad monkey ship

Guybrush in front examine Monkey Figurehead.

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Guybrush dive into ocean depths searching Monkey Figurehead.