The hatch is located in Wailing Woods, a rural and relatively low-traffic area of the Fortnite map, right next to the hedge maze.
So far, no one has been able to open the hatch and see what's inside, including an intrepid Reddit user who dug into the game's coding to see what hints they may contain— but was ultimately unsuccessful.
Curious players will simply have to wait for Epic Games to reveal what's down the hatch, or for some other adventurous player to uncover its mysteries.