Search the indicted location on any difficulty to find the corresponding dungeon. Some hidden dungeon locations are randomized —- they will not appear every time you enter a zone, and the zones may need to be replayed multiple times to find the hidden entrance.
Arch Haven: It is found in the Pumpkin Pastures zone. You will need to revisit Pumpkin Pastures often for it to appear. Look for a ship that spawns on the map. It may take 5-7 replays to find it.
Creepy Crypt: It is found in the Creeper Woods zone. Look for an optional path down the forest to an opening with a small stone structure — interact to unlock the interior. This should appear every time. Keep your map open to see the hidden forest path.
Soggy Caves: It is found in the Soggy Swamp zone. It is located down an optional path that leads to a temple building, similar to the Creepy Crypt. This one appears more rarely —- check how many secrets are on your map. If there are less than three, the entrance will not spawn.
Underhalls: It is found in the Highblock Halls zone. Keep your map open, and reach an area with two hanging shields. Interact with a shield to unlock the path to the hidden area. You will find it as you explore the main path.