In 1991, Doug Bower and Dave Chorley announced that they had been making crop circles for about fifteen years. They said that they would flatten the wheat with a steel bar, planks, and ropes. As they gained more experience, they would create more complex figures. They became angry when other crop figures began appearing in different areas by copy cats. The two men eventually became bored of the prank and confessed, but none really paid attention because crop circles created by aliens is much more interesting than crop circles created by two men. Crop circles continued, and confessions by other people who created crop circles were ignored.
So, clearly crop circles were not created by UFO's landing on Earth, they were just a hoax.
Source: The Demon-Haunted World by Carl Sagan