junction of a tributary, with mountains rising up steeply to enclose it on all sides. Similarly, the temple structure itself isnt designed to be spectacular or visible from a distance, but is concealed from all sides behind high walls. The approach to the site would have been through a narrow ramped entrance in these walls, whose distinctive feature was that they were studded with 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 a ceremonial complex which was reworked and expanded for nearly a thousand years, its last and largest elements dating to around 200BC. The basic arrangement is the by now traditional one of plaza and step pyramid, but these are adorned with far more complexity than their predecessors. Many lintels, columns and stelae 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 scale of the site; 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 sites formative period; the most remarkable is a human figure in a state of feline transformation, bristling with jaws, claws and snakes, and clutching an unmistakable San Pedro cactus like a staff or spear. Beneath this figure - the Chaman, as hes 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 which lead up from the circular plaza to the old pyramid, at the top of which is the familiar altar-like platform. But at the back of this platform is something entirely unfamiliar: a pair of stone doorways disappearing into the darkness inside the pyramid itself. These lead via steps down into tunnels around six foot 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 disorient and cut out the daylight, zig-zagging into