What archaeological phenomenon or mystery of the hispanic world are you most interested in and why?

Deep in the jungles of Central America and northern South America belonged an ancient unnamed civilization that flourished from approximately 700 AD until the Spanish arrived in the early 1500s.  Unlike the Mayans to the North and the Incas to the South, this civilization left little behind – no pyramids, megalithic structures, stone cities or monuments – their dwellings and possessions were typically made of biodegradable material.  As a result, much mystery has surrounded this ancient pre-hispanic culture.