Spermwhales will migrate based on the season they are in. In the coldsnap they migrate south, while in everbloom they head north.
Multiple players swallowed by a whale will all appear in the same stomach.
Should not be confused with the Dead Whale Exploration Events.
They are relatively rare, but like with other animals, you just have to sail the ocean shore and eventually you will find one or a few together (they sometimes swim in pods). When you've found one, you need to swim into their mouth. There will be a small place where you can dismount your canoe and enter the main part of the whale - its' intestines. The intestines is a large labyrinth with lots of dead ends. Piles of excrements will be blocking your paths. You will have to clean your way through them to get to the exit. Digging through the piles will add insanity, which will very rapidly add up in Madness levels, unless you do something to lower your insanity. The exit looks the same way as the entrance. You will need to swim without a canoe when exiting the spermwhale, because it's impossible to mount a canoe once already swimming.