Chichen Itza · Mexico · 20.683056, -88.568611 · PYR record in the living atlas
In the late afternoon around each equinox, the stepped northwest corner of El Castillo throws a chain of triangular shadows down the balustrade of the north staircase. The shadows meet the carved serpent head at the foot of the stairs and the pyramid grows a body of light. Tens of thousands of people stand on the grass to watch it, most of them told that the Maya built the pyramid to do this on exactly that day. That last part is the piece worth examining.
The serpent has a surprisingly recent birthday
The effect is real. You can photograph it. What is contested is whether it was designed. The observation entered print through Jean-Jacques Rivard in 1969 and was popularised by Luis E. Arochi's 1976 book on the pyramid as a solar symbol. Before that, nobody appears to have written it down, and no colonial or Maya source describes it.
The archaeoastronomers Ivan Šprajc and Pedro Francisco Sánchez Nava have argued at the Palenque Round Table that the phenomenon as a piece of deliberate Maya design is a modern construction, dating from those few decades rather than from the ninth century. Their strongest evidence is not rhetorical, it is temporal. The light-and-shadow display can be watched, without meaningful change, for several weeks either side of the equinox. A building tuned to a single day should fail conspicuously a fortnight off it, and this one does not.
Behind that sits a larger finding. Šprajc and Sánchez Nava's survey of orientations across more than a hundred Maya sites points to builders dividing the year at intervals that are multiples of 13 and 20 days, the counting units of the Mesoamerican calendars, rather than at equinoxes. The equinox is a European way of cutting the year. It is not obvious that Maya astronomers used it at all.
Our position: go and watch the serpent, it is one of the finest sights in the Americas, and hold the intentionality question open. The honest version is that a stepped mass with a northerly staircase will cast serrated shadows on the balustrade over a broad span of the year, and that we cannot currently show the Maya were aiming at that. The tour guides are not lying. They are repeating a claim made in the 1970s that nobody made them defend.
One practical consequence, since it is the sort of thing people plan flights around. Because the display holds for a span of days on either side of the equinox rather than switching on for one afternoon, the days before and after give you the same phenomenon with a smaller crowd. They are not a degraded version of it.
The other effect, and the same problem
Clap your hands at the base of the north staircase and the pyramid answers with a descending chirp. The acoustic engineer David Lubman reported this in 1998 and proposed that it imitates the call of the quetzal, a bird of enormous significance in Maya iconography. Nico Declercq later modelled it and found the mechanism: the regular tread-and-riser spacing acts as a frequency filter on the handclap, emphasising some frequencies and suppressing others, and the returned tone falls in pitch. The physics is not in doubt. Declercq's own view of the quetzal reading, though, cooled after he heard comparable chirps from ordinary staircases elsewhere. Two effects, one question, and the same answer: demonstrable, undated, and not yet shown to be intended.
The Great Ball Court carries a third one. The court runs about 168 metres long, with stone rings projecting high on the side walls and carved relief panels along the base showing ball-game ritual, including a decapitation. A handclap at one end of the court is clearly audible at the other. Whether that was designed is not established. It is the same shape of question again: the effect can be measured, the intention cannot.
El Caracol, where somebody did the measuring
Everything above concerns effects that nobody has measured an intention behind. There is a building at Chichen Itza where the measuring was done, and it is not El Castillo. El Caracol is a cylindrical tower standing on a platform, named for the spiral staircase inside it, the snail, and built in at least two distinct phases that are still visible in the platform below it.
In 1975 Anthony Aveni, Sharon Gibbs and Horst Hartung published sub-degree measurements of the tower's window openings and platform edges in Science. The openings correspond to the extreme northerly and southerly setting positions of Venus, to equinox sunset, and to moonset at a major lunar standstill. Those were the first sub-degree alignment measurements published for any Mesoamerican building. That is why El Caracol is called an observatory rather than merely described as looking like one.
The contrast with the serpent is the point of putting the two side by side. One claim has bearings behind it and a journal citation. The other has a photograph and fifty years of repetition. Both are about the same city and the same century, and only one of them has been tested.
A calendar you can climb, counted honestly
The pyramid is about 55.3 metres square at the base and 24 metres to the roof of the summit temple, or about 30 metres counting the temple itself, in nine terraces of local Yucatan limestone. Each of the four staircases carries approximately 91 steps, and 91 times four plus the top platform gives 365. That is the arithmetic everybody quotes. The caveat that almost nobody quotes is the word approximately: the south staircase is badly eroded, published counts are not perfectly consistent, and the total is a reconstruction rather than a tally you can walk. It remains a good candidate for deliberate design, because this is a culture that tracked Venus with real precision and encoded counts elsewhere. It is not a measurement. Nor has anyone shown by excavation or epigraphy that the correspondence was deliberately encoded as a calendrical statement, rather than following building conventions this pyramid shares with others in Mesoamerica.
Two pyramids and a flooded cave
The building is nested. Tunnelling that began in April 1931 opened an earlier temple sealed inside, roughly 33 metres wide and 17 metres high, with a chacmool figure and a red jaguar throne inlaid with jade still in place. Credit for that work is often given to Mexican archaeologists alone and sometimes to the Carnegie Institution of Washington alone. It was joint: Carnegie applied to the Mexican government for permission in 1924 and worked from 1927 with Mexican archaeologists, and the tunnel and its chambers were opened over the following years, the chacmool in 1935 and the red jaguar in 1936. Then, from 2016, a team from UNAM's Institute of Geophysics using three-dimensional electrical resistivity tomography reported evidence of two earlier construction phases within the visible pyramid, implying a third and innermost structure about 10 metres high at the core.
The same technique found something underneath. A survey published in Scientific Reports in 2018 mapped a water-filled karst cavity below the pyramid, about 20 metres north to south and 16 metres east to west, extending more than 20 metres deep and capped by a limestone lid roughly five metres thick. The authors argue the Maya knew it was there and chose the spot for that reason, since every earlier construction phase is centred on it. The city's own name supports the reading. Chich'en Itza means something close to at the mouth of the well of the Itza. It was named for its water, and its greatest building appears to sit on a hidden one.
Cosmic rays, starting this summer
The most interesting thing about El Castillo right now is that the question is live. A project led by Arturo Menchaca Rocha of UNAM's Institute of Physics and Edmundo García Solís of Chicago State University, approved by INAH and funded by UNAM and the United States National Science Foundation, is scanning the pyramid with muon radiography: tracking cosmic-ray particles as they pass through stone, so that voids show up as density shortfalls, without a drill touching the building. Detectors go into the existing tunnels on the north and south sides. The team will first image the two known interior chambers to prove the method works, then map the whole mass. The work is scheduled for summer 2026 and expected to take about six months. Nobody knows what it will show. That is what an open question looks like when it is being properly attacked.
Who dug it, and how much is still buried
John Lloyd Stephens and Frederick Catherwood reached Chichen Itza in 1841 and produced the descriptions and engravings that put the site in front of an English-reading public. They did not excavate it, and 1841 is often quoted as the start of excavation as though they had. Formal excavation began in the late nineteenth century. The first extensive campaigns were those of the Carnegie Institution of Washington from the 1920s to the 1940s, under Sylvanus Morley among others, which cleared, mapped and partially restored El Castillo, the Temple of the Warriors and the Great Ball Court. INAH has overseen the site since the middle of the twentieth century.
That restoration history carries a cost, and it is a chronological one. The order in which the major buildings went up, and how the ceramic sequence of the earlier Chichen Viejo sector relates to the later one, are still being refined, and heavy reconstruction has made that harder rather than easier.
How big the city was is also unsettled. Published estimates of the mapped urban extent run from about 25 to more than 30 square kilometres, depending on where the survey boundary is drawn. Figures near 10 square kilometres circulate widely; they may describe the ceremonial core rather than the mapped settlement, and neither number should be quoted as though it were fixed. Ground-penetrating radar across the Great Plaza in the 1990s found subsurface cavities and a buried causeway, which is a fair reminder that what has been mapped above ground is not the whole of it.
The site is also still producing text. In 2023 the INAH archaeologist Lizbeth Beatriz Mendicuti Pérez found a carved stone disk at the Casa Colorada bearing legible hieroglyphs and figures associated with the ball game. The project archaeologist Francisco Pérez described it as the first complete hieroglyphic text recovered at Chichen Itza in over a decade. In February 2024 the Gran Museo de Chichén Itzá, a new site museum, opened near Pisté.
Who the well actually took
One correction that has already landed. From the sixteenth-century Spanish accounts onward, and hardened by early twentieth-century dredging of the Sacred Cenote, the story of sacrifice at Chichen Itza became a story about young women and girls. In 2024 a genomic study published in Nature analysed 64 subadults from a chultun, an underground chamber, beside the cenote. Every individual sequenced was male. About a quarter were closely related to another individual in the deposit, and the group included two pairs of identical twins, a rate far above anything a normal population produces by chance. The chamber was in use for at least 500 years. The authors connect the twins to the Hero Twins of the Popol Vuh, who descend into the underworld, and note that a chultun is itself an entrance to it. The city also names names: lords such as K'ak'upakal appear in its own hieroglyphic inscriptions in the late 800s. The record was there. The popular story simply was not built from it.
The cenote outlasted the city as a place of offering. It went on receiving deposits into the Spanish colonial period, and the sixteenth-century Spanish accounts describe a city already ruined and still venerated.
What our engines found
We ran the same test here as everywhere else, with the settings fixed in advance and identical at every one of the fifty sites we cover: a 200 km search radius, at least 12 km between sampled sites, 15 sites, a 2 per cent tolerance, and 2,000 Monte Carlo runs against a null model resampled from real catalogued sites in the same region. The settings are frozen because the answer moves if you tune them, and tuning them after seeing the answer is exactly the trick we object to elsewhere. Chichen Itza returned 34 golden-ratio pairs where chance predicts 35.1, giving p = 0.5912 (run on 1 October 2026).
That is almost exactly what chance would predict, and it is not significant. The control points the same way: the control ratio pi/2 scored 1.17 times its expected count against phi's 0.97, so nothing here singles out the golden ratio.
We publish the misses because a tool that only reports its hits is not measuring anything. Every figure above is reproducible from the same coordinates, the same settings and the same seed. When the catalogue grows, the figures can move, so we rerun these tests and update this section rather than leave an old number standing.
One limit worth stating plainly, because it is the one most often glossed over. This test compares positions between monuments. It says nothing about any building's own proportions, its height, base, slope or perimeter. That is a separate question, and at several sites a genuinely open one.
Explore it yourself
Open the Chichen Itza record in the living atlas, see it among its Yucatan neighbours, set the sky to an equinox date and rerun the regional test against our numbers. The Chichen Itza Site-Emblem T-Shirt carries El Castillo with the serpent head, the site geometry and the true coordinates.
Sources worth reading
Aveni, Gibbs and Hartung in Science (1975), the Caracol measurements. Aveni, Dowd and Vining in Latin American Antiquity (2003). Aveni, Skywatchers of Ancient Mexico (2001), for the wider Mesoamerican picture. Šprajc and Sánchez Nava on Maya orientations and the equinox question. Lincoln's 1990 Harvard dissertation on ethnicity and social organisation at the site. Schele and Freidel (1990). Coggins and Shane (eds., 1984) on the material dredged from the Sacred Cenote. Tozzer (1957). For the recent work, the 2018 paper in Scientific Reports on the cavity beneath El Castillo and the 2024 paper in Nature on the chultun burials.