https://www.techradar.com/news/sea-of-thieves-release-date-news-and-features

Update: Sea of Thieves final release date is floating closer and developer Rare has now offered a closer look at the game's Skeleton Forts. These forts are part of the game's "emergent" events like storms and shipwrecks, meaning you'll come across them in different places at unpredictable times. 

Inactive Skeleton Forts were already visible in the Sea of Thieves beta but when the game is launched for real they'll come to life. Players will know a cursed skeleton fort event is about to begin because a huge skull cloud will emerge over the fort and alert players from all over the map where to go. As you'd expect skeleton forts house hoards of unfriendly pirate skeletons.