All human civilizations go through a cycle of rise and fall. Before modern scientific archaeology, ruins and obscure clues led to the myth of "lost civilizations." Perhaps the most famous of these is Atlantis, first described by Plato as having sunk into the sea. While Indiana Jones and the Lost Civilization may make for a good campfire story, there are in fact, ancient civilizations that fell under circumstances still not fully understood.
The Maya are a classic example of a lost civilization. Mayan civilization peaked in the first millennium CE, but after its fall its cities and temples were swallowed up by the Central American rain forest. At its height, Mayan civilization stretched across the Yucatan, from Belize and Guatemala to Mexico, but its cities were abandoned before Europeans arrived, leaving only a few mysterious pyramids in the jungle.
Another classic lost civilization is that of the Indus Valley, also known as Harappan civilization. Located in modern-day Pakistan and India, the Harappan cities, such as Mohenjo-Daro, contained as many as five million people. Speculation is that it became too big to remain sustainable. Easter Island was home to another lost civilization. Famous for its giant stone statues of human figures, called Moai, Easter Island was abandoned when the people depleted all of the island's trees, leaving only the iconic statues.