Phatt Island Waterfall

From Monkey Island wiki

The Phatt Island Waterfall lies at the base of a small river on Phatt Island. On the far side there is a small island where Rum Rogers built a cottage after his retirement, where his son continues to live. At the top of the waterfall is a pump which can be manipulated with a monkey wrench, stopping the flow of the water and revealing an underground tunnel that leads to the small offshore islet. This tunnel resembles the tunnels beneath Dinky Island (and Monkey Island) where Guybrush and LeChuck face off at the end of the game.