SIT on the steps of Mexico's El Castillo pyramid in Chichen Itza and you may hear a confusing sound. As other visitors climb the colossal staircase their footsteps begin to sound like raindrops falling into a bucket of water as they near the top. Were the Mayan temple builders trying to communicate with their gods?
The discovery of the raindrop "music" in another pyramid suggests that at least some of Mexico's pyramids were deliberately built for this purpose. Some of the structures consist of a combination of steps and platforms, while others, like El Castillo, resemble the more even-stepped Egyptian pyramids.
Researchers were familiar with the raindrop sounds made by footsteps on El Castillo - a hollow pyramid on the Yucatán Peninsula. But why the steps should sound like this and whether the effect was intentional remained unclear.
To investigate further, Jorge Cruz of the Professional School of Mechanical and Electrical Engineering in Mexico City and Nico Declercq of the Georgia Institute of Technology compared the frequency of sounds made by people walking up El Castillo with those made at the solid, uneven-stepped Moon Pyramid at Teotihuacan in central Mexico.
At each pyramid, they measured the sounds they heard near the base of the pyramid when a student was climbing higher up. Remarkably similar raindrop noises, of similar frequency, were recorded at both pyramids, suggesting that rather than being caused by El Castillo being hollow, the noise is probably caused by sound waves travelling through the steps hitting a corrugated surface, and being diffracted, causing the particular raindrop sound waves to propagate down along the stairs (Acta Acustica united with Acustica, DOI: 10.3813/AAA.918216).
El Castillo is widely believed to have been devoted to the feathered serpent god Kukulcan, but Cruz thinks it may also have been a temple to the rain god Chaac. Indeed, a mask of Chaac is found at the top of El Castillo and also in the Moon Pyramid. "The Mexican pyramids, with some imagination, can be considered musical instruments dating back to the Mayan civilisation," says Cruz, although he adds that there is no direct evidence that the Mayans actually played them.