Swiss Cheese Alps
From the Super Mario Wiki
The Swiss Cheese Alps are a bizarre-looking mountain range, with the mountains in it being green and filled with holes, giving them a slight resemblance to Swiss cheese. A small village and Dr. Koopenstein's tower and laboratory are located here.
In The Super Mario Bros. Super Show! episode "Koopenstein", Mario, Luigi, Toad and Princess Toadstool traveled to the Swiss Cheese Alps to defeat Dr. Koopenstein and Mouser at the behest of the villagers and the village mayor.