https://www.gamespot.com/articles/fortnite-adds-new-secret-underground-areas-as-part/1100-6458654/

Epic Games has provided Fortnite: Battle Royale players with a lot to digest in the big Season 4 update, and not all of is apparent from the patch notes. Your first post-update Battle Bus trip over the island will reveal some of the big changes, like the comet laying waste to Dusty Depot, which has resulted in a massive crater. But as fans are digging in further, they're finding there's much more to find on the island that you might assume at first.