El Castillo (The Temple of Kukulkan) at Chichén Itzá. It’s built so that precisely half of the temple is in light and the other half in shadow on the spring and fall equinoxes. During this time, the sun aligns with the pyramid, creating a shadow that appears to descend the main staircase. This shadow creates the illusion of the feathered serpent god Kukulcán descending from the sky.