Pokémon Red and Blue/Seafoam Island

'The way through the Seafoam Island caves is full of twists and turns, and you'll have to puzzle your way through to reach the south side of the island. Use the maps on this page to negotiate your way through the treacherous underground pathway. If you leave the cave before you find the southern exit, all of the puzzles will reset.'

Only the strong may enter
Don't even enter Seafoam Island unless you have a Pokémon who knows the Strength technique. You'll need his might to knock the two sets of boulders on the first floor and the third basement into the lower floors, blocking the current so you can swim safely.

The two boulders on the first floor block the current on the third basement level, which makes it easier to get around but is not necessary for catching, the only actual prize in this dungeon. It's the two in the third basement that are important: Knock the other boulders out of the way so you can get to the ones in line with the holes (if you mess up, leave this floor and they'll reset), then jump in after them, swim to the island in the northwest, and save the game before you fight Articuno. You should bring a lot of Ultra Balls, but this fight will be much easier than the last one, thanks to Zapdos' ability to knock Articuno near the red zone with just two Thunder Shocks.