About 400m southwest of the Gran Plaza is El Mundo Perdido (Lost World), a complex of 38 structures with a huge pyramid in its midst, thought to be essentially Preclassic (with some later repairs and renovations). The pyramid, 32m high and 80m along the base, is surrounded by four much-eroded stairways, with huge masks flanking each one.

The stairway facing eastward is thought to have functioned as a platform for viewing the sun's trajectory against a trio of structures on a raised platform to the east, a similar arrangement to the astronomical observatory at Uaxactún. Tunnels dug into the pyramid by archaeologists reveal four similar pyramids beneath the outer face; the earliest (Structure 5C-54 Sub 2B) dates from 700 BC, making this pyramid the oldest Maya structure at Tikal.

A smaller temple to the west, dating from the Early Classic Period, demonstrates Teotihuacán's influence, with its talud-tablero (stepped building) style of architecture.