Situated on Route 20 of Kanto, Seafoam Islands are a pair of islands that divide Route 20 into two separate segments. Hence, to get to the other end of the route, it must be passed through. This large cave comprises of 5 floors, where the extremely rare Ice and Ground-type Swinub can be found.
Besides the basic ladders that helped players to move across different floors, there are also a number of holes on the floor where players can drop through to end up in some specific locations labeled in the maps below, with a green background. For the shortest route to move from Route 20 East to Route 20 West, drop down Hole 13, 15 and 17, then take Stair 9, 7 and 3. To travel from Route 20 West to Route 20 East, players will instead take Stair 3, 7, and 9, then surf across the water to take Stair 8, 5, and 1.
Articuno, one of the three legendary birds can be found inside this cave but you must first obtained the Feather of Articuno. This item can be dropped from any wild Pokémon in here as long as your leading Pokémon is an Ice-type and alive. To encounter Articuno, give the feather to your leading Pokémon and there will be an extremely small chance for the level 85 Pokémon to appear.
Seafoam Islands F1
Wild Pokémon of Level 15 - 25 are encountered here: