Pachamanca is the Quechua language word for a traditional Andean cooking technique, in which vegetables and meats are placed underground with very hot stones and slowly roasted. ("Pacha" means "earth" in Quechuan, and "manca" means "pot"). You can see a real pachamanca in action in this interesting video from Living in Peru, which explains the process in detail. Pachamanca can also be cooked over the fire in a sealed clay pot, which is called pachamanca in olla.
Typical components of a pachamanca include several varieties of potatoes, assorted meats such as pork, lamb, and quinea pig, habas (large lima beans in the pod), tamales, and corn on the cob. The ingredients are layered with herbs such as huacatay (a local mint-like herb) and oregano and covered with very hot stones. This cooking method dates back to the time of the Incan empire, and is still very popular today. In fact, as you drive near recreational areas in Lima on the weekends, waiters will stand in the street and offer forkfuls of their pachamanca as you drive by, hoping to lure you into their restaurant.
By the way, if you are interested in these kinds of traditional cooking techniques and recipes, be sure to visit the website of the Center for Foods of the Americas, a research center focused on Latin American cooking from the Culinary Institute of America. It is a wonderful resource for Latin American culinary history. Don't miss the excellent video about how to nixtamalize of corn with limestone, which is an ancient chemical process for dehulling corn (which apparently also improves the nutitional qualities of the corn). This is how hominy and masa are made. The chef shows how the corn is ground into masa, for making tortillas and tamales, and demonstrates how to make tortillas.