The Mandy Pyramid is the ruins of an ancient pyramid-shaped temple that forms a deep underground maze. It is discovered during the events of Monster Rancher Battle Card, as a secret entrance to the maze is found right outside one of the Aurora city walls. A team of experts is brought in to investigate, and in the end it turns out that one of the city elders has known about the ruins all along and has been keeping it a secret. Cox helps you to explore them.
This undergroud set of ruins is actually mentioned again as one of the possible ranch locations in My Monster Rancher, called the Sealed Underground Ruins.