Mount Spew is Kinkow's volcano.
As Mikayla described in the episode 'Return of the Kings', the volcano is a symbol of how happy the island is and how the Dark side is weak. It has several wind tunnels, including the one where Mikayla and her friend Candis fell into in the episode The One About Mikayla's Friends.
Mt Spew threatened to destroy Kinkow, because Brady and Boomer drove the King Cruiser into Giki-Kiki and caused the sacred ruby to fly out of the statue. (Return of the Kings)
Boomer made a wish on his and Brady's birthday to see the volcano's oracle and find out who the older twin was. (Journey to the Center of Mt. Spew)
The boys fell into a cave in Mt Spew and found a creature that draws Kinkow's former royalties, including the boys' parents. (Kings of Legend)
Wind tunnels manage to get to the boys because Candis and Mikayla fell into one. (The One About Mikayla's Friends)
Kaita the Bat Rider was thrown into the volcano by Boomer and Boz, killing him, and fulfilling the prophecy. (Long Live The Kings)