Chichén Itzá is a Maya city in the flat scrub of Mexico's Yucatán peninsula, built around a pyramid with 365 steps, and twice a year, on the equinoxes, the afternoon sun throws a snake of shadow down its northern staircase to meet a carved stone serpent head at the bottom. The pyramid is El Castillo, thirty metres of stepped stone with a temple on top, and it is a calendar as much as a building: four staircases of 91 steps plus the platform make the days of the year. Clap once at the foot of the north stairs and the echo comes back as a sharp chirp that acousticians have matched to the call of the quetzal, the bird the Maya held sacred. It is not solid. Inside is an older pyramid, and inside that a temple where archaeologists found a red-painted jaguar throne studded with jade; and in 2015 a scan found a water-filled cenote about twenty metres across under the foundations, so the pyramid was most likely built over a sacred sinkhole on purpose.
Water is the key to the whole city. Yucatán has no rivers; its water runs underground and shows itself only where the limestone roof falls in, and the Sacred Cenote, a wide sinkhole at the end of a raised causeway, was where the Maya threw gold, jade and people to the rain god Chaac. The Great Ball Court is the largest in the ancient Americas, 168 metres long with walls eight metres high, and a word spoken at one end can be heard at the other. Beside the Temple of the Warriors stand the Thousand Columns, rows of carved pillars that once carried a wooden roof. El Caracol, a round tower with slit windows, tracked Venus, a planet the Maya tracked with startling precision. The city was at its height around a thousand years ago, and the land under the pyramid belonged to a private family until the state bought it in 2010. On the equinox the north lawn fills by mid-afternoon, and when the shadow serpent finally moves, the crowd goes quiet.