The Funderdome is a map in Plants vs. Zombies: Battle for Neighborville. It is where Battle Arena is held. It also the location of the boss fight against Olds Cool in Town Center, when playing as a plant.
As of recently, the Funderdome map was changed to be bigger. It also alternates between having water, hot cheese, and bouncy ooze as an obstacle.
Olds Cool Boss Fight
The Funderdome when a player fights Olds Cool is much different from the one in Battle Arena.