In the climatically stable Nazca desert of Southern Peru are a series of giant geoglyphs (a design marked on the ground) which are best viewed from the air. Most of the markings are very straight lines across the flat desert floor but others show figures of plants and animals such as Monkeys, Jaguars, birds and even people 100's of m across. The consensus is that they were created between 500 BCE and 500 C and were made by creating trenches about 10-15 cm deep revealing the lighter subsoil under the red-oxide desert covering.
For many years no one knew how they were made or why. As is usual, mysterious things attract weird and wonderful hypotheses, consequently they featured in Erich von Däniken's infamous 'Chariots of the Gods book in claiming they were made by ancient astronauts visiting Earth as humans would not have had either the tools to create them, or the ability to see them from above without flight. He suggested the straight lines were landing strips and the geometric designs there to attract the alien visitors. One figure is known as 'The Astronaut' and has a very superficial resemblance to a space-suited figure
Skeptic Joe Nickel was able to recreate the lines using tools and techniques available to the people at the time and archaeological finds of stakes and ropes marking the straight lines have been found. The shapes are also visible from nearby mountains. No one can definitively say why they were created - humans have often created massive drawings in the landscape, see the chalk figures across England - but we know how they were made.