Valley of Bowser
From the Super Mario Wiki
The Valley of Bowser is the final area of Super Mario World before reaching the final boss, Bowser. The main way into the Valley of Bowser is by beating the Sunken Ghost Ship and entering the massive rock carving of Bowser's head that rises out of the sea. However, the player can also catch a glimpse of this area by using the warp pipe found in Donut Plains to access Donut Secret 2 (the level upon the cliff on the left). Additionally, the player can enter the valley using the secret warp pipe on Chocolate Island, which leads to Chocolate Secret (the level at the bottom of the map).
There are frequent flashes of lightning in the background despite this level being accessed via a subterranean cavern. Various enemies, though mostly Koopa Troopas and cave-dwelling enemies, can be encountered here. What is presumably the Valley of Bowser, renamed the Valley of the Koopas, appears in the Nintendo Adventure Books Dinosaur Dilemma and Flown the Koopa. Depicted as a barren, putrid, desert-adjacent wasteland filled with garbage and hostile creatures such as Munchers and Dino-Torchs, the area contains Bowser's summer home, where he holds a party celebrating his granting of the Snowbell Prize in Dinosaur Dilemma.
Note that levels are listed in the order the player access them. Also, levels with an asterisk (*) feature more than one exit.
Red Switch Palace: There is an unused Switch Palace to the left of Valley of Bowser 3. Since it leads to an "empty" level, it is probable that it was scrapped early in development. Beating the Secret Exit of Valley of Bowser 3 would cause Mario to walk to this switch palace, except that Valley of Bowser 3 has no secret exit, and there is no path for Mario to walk on. This glitches the game, causing the player to be trapped in a endless Bonus Game.
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