The Valley of Bowser was a underground cavern in the center of the Dinosaur Land inhabited for Monty Moles, served as King Bowser and his family's realm after his defeat in the Koopa Kingdom. When the airship crash in sea, this underground became the Koopa Troop's refuge where Bowser planned to rule Dinosaur Land, kidnap Princess Peach Toadstool and steal Yoshi's eggs. How the Dinosaur Land have no technology, Bowser use his magic as energy in his new Castle and Larry also build his small castle in this place. When plumbers take magic ball from Bowser's sunken airship, a valley entrance rise up out of the water. From outside, is a islet with Bowser head sculptured; inside is a cave with lakes, a Boo House, Reznors' fortress, ice caverns and mysterious lightning in background. It was here that plumbers had to battle with Bowser when the same uses his Clown Car to be defeated and save princess.
- What is presumably the Valley of Bowser, renamed the "Valley of the Koopas", appears in the Nintendo Adventure Books titled "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.
- There is a red-colored, 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.
- It is sometimes known as the "Valley of Koopa.