The ancient Maya, whose early settlements date back to about 2,000 B.C., lived in present-day southern Mexico and northern Central America. As a civilization, they are recognized for their sophisticated calendar systems and hieroglyphic writing, as well as their achievements in areas such as agriculture and architecture.

Around A.D. 250, the Maya entered what’s now known as the Classic Period, an era in which they built flourishing cities with temples and palaces, and population size peaked. However, by the end of the Classic Period, around A.D. 900, almost all of the major cities in what was then the heart of Maya civilization—the southern lowlands region, in present-day northern Guatemala and neighboring portions of Mexico, Belize and Honduras—had been abandoned.

The collapse didn’t happen all at once; instead, it’s believed to have occurred over time from place to place, between about the late 8th and 925. Exactly why any of this transpired, though, is a mystery.

Scholars have suggested a number of potential reasons for the downfall of Maya civilization in the southern lowlands, including overpopulation, environmental degradation, warfare, shifting trade routes and extended drought. It’s likely that a complex combination of factors was behind the collapse.

What is certain is that the Mayans didn’t disappear in the aftermath of the collapse. Instead, cities in the northern lowlands region, such as Chichen Itza and later Mayapan (both located in present-day Yucatan, Mexico), rose to prominence. The Maya also established cities in the highlands region, such as Q’umarkaj (in present-day Guatemala).

The Spanish conquistadores arrived in the early 1500s and the last independent Mayan city, Nojpeten (in present-day Guatemala), fell to Spanish troops in 1697. The ancient cities were largely forgotten until the 19th century, when their ruins started to be uncovered by explorers and archeologists. Today, the Maya continue to reside in their ancestral homelands in Mexico, Guatemala, Belize, Honduras and El Salvador.