Fortuna Caverns are a series of caverns on Isle Fortuna.
There are three entrances to access Fortuna Caverns. The cellar in Henry's Promenade, the cellar in the Head Privateer Office, and the small beach on the far side of Isle Fortuna.
Entering from the Head Privateer Office, which is the main entrance, you will come across a wooden trapdoor guarded by Quartermaster Reilly. When you enter, you climb down a set of stairs into a dirt tunnel. Signs of excavation are all over, and there are many wooden beams. Following the tunnel a short distance, you reach another wooden door. Entering this door brings you to a mining elevator (similar to the one on Raven's Cove). This is the main entrance to Fortuna Caverns.
Taking the elevator down, you reach a small survey camp with a low ceiling. Further investigation reveals there are two Royal Navy NPCs (non-hostile). They seem to be looking at a map. There is also a miner in the corner, who is sitting on a chair, and looking from side to side.
Walking further down the low-ceiling cavern, you start to notice many paths winding off. These paths lead to different areas of the cavern. Some lead to different survey camps, some lead to excavations. The other two paths lead to the other exits. There are also several NPCs walking around in the cavern.
Fortuna Caverns were evacuated when the Navy caught word that there was a mighty treasure below the island.