The event starts with a warning in the chat: a submarine will soon be passed near the island.
A submarine will appear on the water when the time is up.
There are 4 floors in the submarine. 2 floors are over the water and 2 floors are under the water.
It is possible to get into the submarine on absolutely any transport. There are 4 outside entrances, 4 underwater entrances, and 2 submarine entrances (added in the Underwater Update).
There are about 50 NPCs...
