Pokémon Ruby and Sapphire/Seafloor Cavern

Seafloor Cavern is a subterranean cave deep beneath the waves. The cavern is somewhat maze-like, and constantly requires the use of Rock Smash and Strength. In a misty cave, you'll discover TM26 (Earthquake). For more details on getting through the Seafloor Cavern check below.

Underwater Entrance
A cramped underwater area where you'll find the lost submarine that Team Aqua/Magma stole. Without much else to do use dive to surface.

Entrance and Entrance Room
Well, you made it, the Seafloor Cavern. Now all that's left it to go inside. The Entrance Room has two Team Aqua/Magma Grunts and three doorways leading to the Entrance, Room 2, and Room 5.