https://www.gamespot.com/articles/fortnite-season-4-introduces-secret-underground-ba/1100-6458654/

It's been a big week for Fortnite. Epic Games released the big 4.0 update, marking the beginning of Season 4 and introducing some significant changes. The most obvious involves the huge crater on the map created by the comet that landed, but the Battle Royale island actually has a lot more going on than meets the eye at first. As players have spent more time exploring, hidden underground bases--seemingly tied to superheroes and villains--have been discovered around the map.