The monumental ruins of Chavín de Huantar, 10,000 ft (3,000m) up in the Cordillera Blanca of the Peruvian Andes, are – officially – a mystery. The vast, ruined granite and sandstone structures – cyclopean walls, huge sunken plazas and step pyramids – date from around 1,000BC but, although they were refashioned and augmented for close to 1,000 years, the evidence for the material culture associated with them is fragmentary at best. Chavín seems to have been neither a city nor a military structure, but a temple complex constructed for unknown ritual purposes by a culture which had vanished long before written sources appeared.

The archæological consensus is that Chavín was some kind of ceremonial focus; some have tentatively located it within a lost tradition of oracles and dream incubation. But the mystery remains profound, and is considerably heightened by the bigger picture that it represents. By most reckonings, and depending on how the term is defined, ‘civilisation’ emerged spontaneously in only a handful of locations around the globe: Mesopotamia, the Indus Valley, China, Mexico, perhaps the Nile. To this short list, especially if civilisation is defined in terms of monumental architecture, must now be added Peru. It was only in the 1930s that Chavín was proposed to be some 3,000 years old, and it’s only recently been recognised that huge ceremonial structures of sunken walled plazas, typically set at the foot of raised stone platforms and pyramids, were being constructed in Peru at least 1,000 years earlier. The coastal site of Caral, only now being excavated, turns out to contain the oldest stone pyramid thus far discovered, predating those of Old Kingdom Egypt. So, the mystery of Chavín is not an isolated one: it was the flowering of a pristine and unique culture, and one which still awaits interpretation.

The emergence of Chavín as a ceremonial centre, probably around 900BC, represents a far more complex construction than its predecessors, and one that adds to their unadorned ruins a spectacular new dimension of symbolic art. But these symbols were not designed for profane eyes. The only images visible from outside the temple complex would have been those which studded the walls: gargoyle-like, life-size heads, some human, some distinctly feline, with exaggerated jaws and sprouting canine teeth, and some, often covered in swirling patterns, in the process of transforming from one state to the other. This process of transformation is clearly a physical ordeal: the shapeshifting heads grimace, teeth exposed in rictus grins. In a specific and recurrent detail, mucus emanates in streams from their noses.

Inside these walls – now mostly crumbled, and with the majority of the heads housed in the on-site museum – there are still substantial remains of the ceremonial complex which was reworked and expanded for nearly 1,000 years, its last and largest elements dating to around 200BC. The basic arrangement is the traditional one of plaza and step pyramid, but Chavín’s distinctive, hallucinatory visuals seethe across the faced stone surfaces: lintels, columns and stelæ are covered with relief carvings, swirling motifs featuring feline jaws, eyes and wings. The initial impression is amorphous and chaotic, but on closer inspection these motifs unfurl into composite images, their interleaved elements in different scales and dimensions, the whole often representing some chimerical entity composed of smaller-scale entities roiling inside it. As the architecture develops through the centuries, it becomes larger in scale, reflecting the increased size of the site as a whole; at the same time, the reliefs gradually become less figurative and more abstract, discrete entities melting into a mosaic of stylised patterns and flourishes.

It was only in 1972 that the most striking of these reliefs were uncovered, on faced slabs which line the oldest of the sunken plazas, running like a frieze around its circle at knee height. These figures are presumably from the site’s formative period; the most remarkable is a human figure in a state of feline transformation, bristling with jaws, claws and snakes, and clutching a hallucinogenic San Pedro cactus (Echinopsis/Trichocereus sp.) like a staff or spear. Beneath this figure – the ‘Chaman’, as he’s become informally known – runs a procession of jaguars carved in swirling lines, with other creatures, birds of prey and snakes, sometimes incorporated into the whorls of their tails.

These reliefs are all carved in profile, and all face towards the steps that lead up from the circular plaza to the old pyramid, at the top of which is the traditional altar-like platform. But at the back of this platform is a new element: a pair of stone doorways disappearing into the darkness inside the pyramid itself. Steps lead down into tunnels around 6ft (1.8m) high and constructed, rather like Bronze Age long barrows, from huge granite slabs and lintels. The tunnels take sharp, maze-like, usually right-angled turns, apparently designed to cut out the daylight and disorient, zig-zagging into pitch blackness. Opening out from these subterranean corridors are dozens of rock-hewn side chambers, some large enough for half a dozen people, others for only one. There are niches hacked in some of the chamber walls, which might have housed oil lamps, and lintels which extrude like hammock pegs. Running through the bewildering network of tunnels and chambers are smaller shafts – some of them air vents, others water ducts which allowed the nearby spring to gush and echo through this elaborately constructed underworld.

Right in the heart of the labyrinth is a stela carved in the early Chavín style, a clawed, fanged and rolling-eyed humanoid form, boxed inside a cramped cruciform chamber which rises to the top of the pyramid. The loose arrangement of stones in the roof above, which form a plug at the crown of the pyramid, has led to speculation that they might have been removable, allowing the Lanzon, as the carved stela is known, to point up like a needle to a gap of exposed sky. Other fragments of evidence from the site, such as a large boulder with seven sunken pits in the configuration of the Pleiades, suggest that an element of the Chavín ritual – perhaps, given the narrow confines around the Lanzon, a priestly rather than a public one – might have involved aligning the stela with astronomical events.

This plaza and pyramid was Chavín’s original structure, but over the centuries more and grander variants were added. There are several shafts, some still unexcavated, which lead down into larger underground complexes, their stonework more regular than the old pyramid and their side-chambers typically more spacious. There’s a far larger sunken plaza, too, square rather than circular and leading up to a new pyramid and surrounding walls on a more massive scale. Whatever happened at Chavín, the architecture suggests that it carried on happening for centuries, and for an increasing volume of participants.

The term most commonly applied to what went on at Chavín is ‘cult’, although elements of meaning might perhaps be imported from other terms like pilgrimage destination, sacred site, oracle or, in its classical sense, temple of mysteries. This is a conclusion partly arising from the lack of any evidence that it represented an empire, or a state power: there are no military structures associated with it, nor centralised labour for major public works like irrigation or housing. During the several centuries of its existence, tribal networks would have risen and fallen, changes in the balance of power apparently leaving its source of authority untouched. Its cultic – or cultural – influence, though, spread far and wide. Throughout the first millennium BC, ‘Chavínoid’ sites spread across large swathes of northern Peru, and pre-existing natural huacas (sacred sites) began to develop Chavín-style flourishes: rock surfaces carved with snaky fangs and jaws, standing stones decorated with bug-eyed, fierce-toothed humanoid forms. People were clearly coming to Chavín from considerable distances, and carrying its influence back to far-flung valleys, mountains and coasts.

Was Chavín, then, a religion? There’s been some speculation that the carvings on the site represent a ‘Chavín cosmology’, with eagle, snake and jaguar corresponding to earth and sky and so forth, and the humanoid shapeshifter, as represented on the Lanzon, a ‘supreme deity’. But Chavín was not a power base that could coerce its subjects to replace their religion with its own: the spread of its influence indicates that it drew its devotees from a wide range of tribal belief systems with which it existed in parallel. It’s perhaps better understood as a site which offered an experience rather than a cosmology or creed, with its architecture conceived and designed as the locus for a particular ritual journey. In this sense, the Chavín figures would not have been deities competing with those of the participants, but graphic representations of the process that took place inside its walls.

This process suggests a function for Chavín’s mysterious architecture: as a site where participants could enter a shared otherworld, and also submit themselves to a highly charged individual vision quest. The sunken plaza might, as the reliefs suggest, have harnessed the consciousness-expanding effects of the San Pedro cactus to a mass ritual of dancing and chanting. The participants might subsequently have ascended the temple steps, perhaps to receive a further plant sacrament: site finds include carved bone tubes typically used for taking a snuff of powdered DMT-containing seeds (Anadenanthera sp.), which produce a brief, intense hallucinatory trance accompanied by streams of mucus from the nose. At this point, the priests would lead them into the chambers within the pyramid where they could experience visions in solitary darkness. Here, the amplified rushing of water and the ecstatic sounds of the unseen participants around them would enclose them in a supernatural world, one where ordinary consciousness could be abandoned, the body itself metamorphosed like the shapeshifting heads on the temple walls, and the world seen from an enhanced, superhuman perspective – analogous, perhaps, to the uncanny night vision of the feline predator. The development of the subterranean chambers over centuries would reflect the logistical demands of ever greater numbers of participants willing to submit themselves to a life-changing ordeal that offered a glimpse of the eternal world beyond the human.