Moleville is a fairly large place home to the Monty Moles. The houses appear to be made of rocks and have metal rooftops. There is also large, green vegetation that grows around.
After defeating Bowyer and running through the Pipe Vault, Mario's team end up at Moleville where Geno can sense another Star somewhere. Before entering, Mario notices Bowser assembling his Koop Troop over the top right side of the place and they leave. Mario doesn't hesitate to enter Moleville Mines to continue his search.
Shiny Stone Side-quest
Mario can actually get the Shiny Stone in this very place. All he needs to do is buy Fireworks (which costs 500 coins) from a Monty Mole in a house and give it to the younger Monty Mole. After that, he can go back to Monstro Town and break the seal on the door to fight Culex.