Bowser in the Fire Sea

Bowser in the Fire Sea, also called Bowser's Lava Sea or simply the Fire Sea, is the second Bowser level in Super Mario 64 and Super Mario 64 DS, located in the basement of the Mushroom Castle. This level is made of concrete and mesh platforms above a huge lake of lava, sharing Lethal Lava Land's background of a vast lava ocean with a fiery sky. Several Bullies and Goombas guard these platforms and the route to Bowser's arena. This level is unlocked by completing Board Bowser's Sub in Dire, Dire Docks, and its own completion unlocks the Mushroom Castle's second floor, which leads to Snowman's Land; Wet-Dry World; Tall, Tall Mountain; Tiny-Huge Island; and Chief Chilly Challenge (DS version only).

The entrance to Bowser in the Fire Sea is found in the same room of the Mushroom Castle as the entrance to Dire, Dire Docks, which requires 30 Power Stars to enter. Before the player obtains the first Power Star from Dire, Dire Docks, the blue portal to the water level will be in front of the entrance to Bowser in the Fire Sea. Once the first Power Star of Dire, Dire Docks is obtained, the blue portal will move to the back of the room, allowing access to Bowser in the Fire Sea. One Power Star can be found in the Nintendo 64 version of the game, while two can be found in the Nintendo DS remake.

Layout
The player starts on a small platform with a metal platform near it. Stepping onto the metal platform will cause it to move. At certain points, the platform will dive under the lava, so the player needs to jump to nearby tilting platforms in order to avoid taking damage. After the moving platform's stopping point is a series of blue platforms that constantly go under the lava, then resurface. Beyond those is a Bully followed by three Goombas situated around a Spinning Heart. A rising pole, circled by an Amp, will help the player reach the tilting platforms above, and the pole at the end of those leads to a cage with an elevator that brings the player up to the next level.

The second level first contains a moving platform that goes through a flame thrower, which leads to a sloping mound. After the slope is a hanging grate the player can use to climb to the other side of the lava pool. Beyond that are two moving platforms, followed by a couple of Bullies, and a tall set of moving interconnected platforms with a Keronpa Ball that will lead the player to the next level.

The final level starts with a stone slope, containing a flame thrower, a Bob-omb, and a ! Block. After the stone slope is a row of sinking and rising platforms that are protected by flame throwers and Amps. Beyond those is a stone structure containing rising and falling poles. Finally, a falling bridge leads to the warp that sends the player to the Bowser fight.

Bowser battle
The tactic to defeat Bowser is the same as that for the fight in Bowser in the Dark World, though he has some additional moves. Bowser will immediately start the battle by jumping up and landing, making the stage tilt toward him. The same will happen if the player throws Bowser off the stage but misses a Floating Mine, causing Bowser to land and cause the stage to tilt. Bowser can also warp to a different spot if the player is close, and charge at the player if they are far away. Throwing Bowser into a Floating Mine will defeat him, granting the player the Big Key to the castle's upper floors.

Missions
Two Power Stars can be collected in this course, with one being exclusive to the DS version of the game.

8 Red Coins
Like in Bowser in the Dark World, there are eight Red Coins to be collected. These are the locations of the Red Coins:
 * Floating above a central hole in a square metal floor with regular coins, found at the end of the metal floor path before the first Spinning Heart
 * Floating above a side of the first tilting platform after the pole circled by an Amp
 * Floating above in a corner of the cage with the elevator
 * Floating below the elevator in the cage. The player must get on the elevator, quickly get off it, and then fall down the hole to get this coin.
 * Floating above the octagonal floor with a Bully on it at the end of the narrow path next to the top of the sloping mound
 * Floating above the third moving interconnected purple platform, indicated by a small shadow. The player must wait for the tall platform set to rise enough to get this coin.
 * Right next to the last flame thrower in the area with sinking and rising platforms on the third level
 * Floating above the last sinking and rising pole before the falling bridge leading to Bowser

After the player collects all eight Red Coins, the Power Star will appear atop a wall shortly before the warp that leads to Bowser himself. The player must wall-jump or use Luigi's backflip to reach it.

Switch Star of Bowser in the Fire Sea (DS only)
Near the first Star is a Star Switch. Upon the player pressing the switch, the Power Star will appear on the far side of a lava pool that the player must quickly backtrack across in order to reach it before it disappears. If playing as Luigi, the player can backflip over the lava to the Star.