They used a slash-and-burn technique for farming
The Mayans created the calendar by watching the solar cycle.
How did the Mayan civilization come to an end?
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They would take down trees and burn the rest to let the nutrients go into the soil.
There are 4 main theories on how the Mayan civilization died out, and those are war, disease, famine, or a disaster or environmental change.
I think disease is the main reason for the Mayans dying out is disease. It was most likely yellow fever or Peregrinus maidis. My reason why is because disease is really easy to spread around, especially if they didn't have the kind of technology we have today.