Machu Picchu
When the Spanish invaded Peru, the Incas left Machu Picchu. Nobody knows for sure why they did that, but some think it was because of diseases from Europe (such as smallpox). The city was left unfinished, most likely due to the Spanish invasion and/or a civil war between the ruling rival Inca brothers named Huascar and Atahualpa. The Spanish never found Machu Picchu or the lost city during their occupation.