One of the challenges to upgrade your Mandalorian skin in Fortnite is to find the Beskar Steel deep in the belly of the shark. If you do, it will net you some more Beskar Steel armor to make your skin all nice and shiny.
The Shark has been a location in Fortnite for some time now and can be found in the northwest corner of the map, above Coral Castle. It is a good idea to drop straight in there from the Battle Bus for this challenge.
Drop into the water near the mouth of the giant stone shark, then swim inside. Go all the way to the back of the ground floor and you will see some stairs on your left. Go up one floor, then turn right towards where the old vault was.
The vault itself is now cracked open and you can get inside with any issues. Inside the vault on the left you will find the Beskar Steel on a shelf. All you need to do is interact with it to collect it, and you will have completed this challenge.
This is the fourth piece of armor that you will be picking up for your Mandalorian skin, and you will be getting awfully close to completing the entire thing.