Tiwanaku collapsed as a civilisation around 1000 AD, probably due to prolonged drought, and was largely in ruin before the Inca empire even began. What remains is still striking: the Akapana pyramid, partly excavated and partly still buried, rises seven terraced levels above the altiplano plain; the Kalasasaya platform contains the Gateway of the Sun, a single block of andesite stone carved with a central figure — the Staff God — surrounded by 48 winged attendants, which appears in various forms across Andean cultures for centuries afterward. The craftsmanship is precise in a way that becomes more impressive the longer you look.
Puma Punku, a short walk from the main temple complex, is the most puzzling section of the site. It consists of enormous H-shaped stone blocks, some weighing over 100 tonnes, that appear to have been precisely interlocked into a structure that was then dramatically destroyed — possibly by earthquake or deliberate dismantlement. The edges and corners are machined-looking in a way that seems implausible for the period. Archaeologists have reasonable explanations, but standing in front of the actual stones, the explanations don't entirely satisfy.
To get there, take a shared taxi or one of the tourist buses that depart from the Cemetery district in La Paz. The journey is 70 kilometres and takes about 90 minutes. Most buses allow two to three hours at the site; for Puma Punku as well as the main complex and the on-site museum, you want at least three hours. Bring sunscreen — the altiplano UV is extreme — and a jacket for the wind.