Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Buried Relic

After beating Stormy Sea, you should receive mail about the Buried Relic. Go to Pokémon square and talk to Lombre then Shiftry to unlock this massive dungeon. Note: do not enter the dungeon until you buy the friend area Final Island.

Warning
This dungeon requires a quicksave before entry, so don't take anything important with you.

The first fourteen floors shouldn't be too hard. You'll face Regirock on the 15th, Regice on the 25th and Registeel on the 35th. Each will drop a part. When you have all three parts, they will fuse to become a music box. Do not use it. Until floor 98, there is a chance of encountering Mew. You can recruit him if you have the Final Island friend area. There are only Golem on floor 99.