There is growing recognition that societies without prominent hierarchies could build large constructions. Scholars are debating what motivated many people to participate in these construction projects. We investigated the site of Aguada Fénix, Mexico, which features the oldest and largest monumental architecture in the Maya area. Using light detection and ranging (LiDAR) and excavations, we documented a site plan composed of nested cross forms built between 1050 and 700 BCE. Its center was marked by a large cruciform cache containing the earliest known directional color symbols in Mesoamerica. The overall pattern consisted of 9- and 7.5-kilometer-long axes delineated by canals and corridors. The builders constructed canals, measuring up to 35 meters wide and 5 meters deep, and a dam to supply them with lake water. Although the canals appear unfinished, this site plan exceeded or rivaled the extents of later Mesoamerican cities. Aguada Fénix was probably designed as a cosmogram, which likely attracted people from a broad area.