To get to Lost Cave in FireRed/LeafGreen, you have to sail to Five Island, Surf north across the Water Labyrinth, and then head east off of Resort Gorgeous.
In order to reach Five Island, you must first get the Rainbow Pass from Celio after obtaining the National Pokédex, and then board the Seagallop Ferries in the Vermilion City Harbor.
You pretty much can’t reach the Lost Cave until post-game. There are a lot of goodies in the cave though, meaning it is well worth the trip once you’ve beaten the Elite Four and grabbed the National Dex.
Specifically, you’ll need to do the following before you can hit up the Lost Cave:
- Get the Tri-Pass from Celio after defeating Blaine
- Defeat the Elite Four
- Receive the National Pokédex from Professor Oak (after catching 60 Pokémon)
- Get the Rainbow Pass from Celio on One Island
The Rainbow Pass is the most important thing here, since you need it to get to Five Island where Lost Cave is located.
Unfortunately you can’t get that pass until you beat the Elite Four and get the National Dex.
How To Get To The Lost Cave
Once you have the Rainbow Pass, reaching the Lost Cave is pretty easy.
You’ll need to have a Pokémon that knows Surf, but I’m assuming you’ve already checked that box by this point.
Step 1: Board the Seagallop Ferry and sail to Five Island.
Step 2: Once on Five Island, head north and Surf across Water Labyrinth to Resort Gorgeous.
Step 3: Sail east from Resort Gorgeous until you reach Lost Cave.
The Lost Cave is hard to miss. It’ll just be a tiny rock formation with an entrance in the front of it directly to the east of Resort Gorgeous.
How To Navigate Lost Cave
There are five items you can grab from the Lost Cave, but getting them is a puzzle. If you take the ladder down into the cave, you’ll meet the Ruin Maniac Lawson.
After you beat him, you can start the puzzle.
Each item has a specific set of directions you need to follow in order to obtain them.
Simply take the “doorways” in the specific order outlined below for each item. Just make sure you always start from the same room as the Ruin Maniac.
Note: if you take the wrong door, then you will find yourself right back at the beginning of Lost Cave and need to restart.
|Lax Incense||East, South|
|Sea Incense||East, North, South, North|
|Max Revive||East, North, South, South, East, East|
|Rare Candy||East, North, South, South, East, West, South, West|
|Silk Scarf||East, North, South, South, East, West, South, East, North|
The first time you follow the directions for Silk Scarf, you will rescue an NPC girl. Just head back to the Lost Cave after and follow the directions again for the Silk Scarf.