Whirlpool

A whirlpool is a dangerous natural phenomenon of the open sea. These gigantic, swirling vortexes can suck in even the biggest ships and eat them whole. Fortunately, they are also so big that it is easy to avoid them, as they move very slowly. Sometimes, steering the ship into a whirlpool can have unexpected effects.

In Ultima III there was only one whirlpool. However, it travelled quickly and always threatened to engulf ships. Directing the ship into it led to the loss of the ship, but would transport its passengers to the lost island of Ambrosia. By Ultima IV, several whirlpools could be found at sea. Pass through one with the ship would result in the ship reappearing in Lock Lake. Some years later, Captain Johne's ship, the Ararat, was swallowed by a whirlpool, sending it into the Underworld beneath Despise. The Avatar had the option of doing the same in Ultima V.

After this time, the whirlpools vanished. Despite being mentioned as endangering shipping in Ultima IX, only a small one is seen, in a cutscene.

Lore
"A huge swirling --WhirlPool-- engulfs you and your ship dragging both to a watery grave!... As the water enters your lungs you pass into Darkness! You awaken on the shores of a forgotten Land Your ship and crew lost to the sea! - entering the whirlpool in Ultima III"

Runes of Virtue
In Runes of Virtue II, whirlpools once again appear in Britannia, and should the hero's ship veer into their maw, they would once again be transported to Cove like in Ultima IV.