The most accessible ancient archeological site in Costa Rica, the MONUMENTO NACIONAL GUAYABO lies 19km northeast of Turrialba and 84km east of San José. Discovered by explorer Anastasio Alfaro at the end of the nineteenth century, the remains of the town of Guayabo, believed to have been inhabited from about 300 BC to 1400 AD, were only excavated in the late 1960s. Administered by MINAE (which also controls Costa Rica’s national park system), Guayabo today suffers from an acute shortage of funds: some of the montículos (stone mounds) are in a poor state of repair and only a small part of the site has been excavated. With the withdrawal of the annual US aid grant, the prospects for further exploration look bleak.

The site, a dairy farm until 1968, is visually disappointing compared to the magnificent Maya and Aztec cities of Mexico or Guatemala – cultures contemporaneous with Guayabo – though it’s well to remember that civilizations should not necessarily be judged on their ability to erect vast monuments. Facing the considerable difficulties posed by the density of the rainforest terrain, the Guayabo managed not only to live in harmony with an environment that remains hostile to human habitation, but also constructed a complex system of water management and social organization, and expressed themselves through the “written language” of petroglyphs.

The mysteries of Guayabo are amplified by today’s site, which lacks anything in the way of information or interpretation; it’s a good idea to hire a guide to help you decipher what can otherwise look like random piles of stone. Either way, it’s best to start at the gloomy exhibition space, which has a model showing how the town would have looked, before heading up to the mirador for an overview; the trail (1.6km) then weaves its way down among the mounds.

Most of the heaps of stones and basic structures now exposed were erected between 300 and 700 AD, though the (still working) aqueducts at the northwestern end of the site are some 2000 years old. Excavations have shown that the Guayabo were particularly skilled in water conducting – look out for the stone tanque de captación near here, where they stored water carried in these subterranean channels from nearby springs.

At the heart of the town is the central mound. Of the 43 montículos that make up the site, this is the tallest circular base unearthed so far, with two staircases and pottery remains at the very top. Guayabo houses were built to a hierarchial system, and it is likely that this was home to the community chief, a cacique, who had both social and religious power. Near the central mound, you can see some of the tombs (known as Tumbas de Cajón, or Drawer Tombs) that have been uncovered in various parts of the site. They were constructed in layers of rock (hence their name) brought from surrounding rivers; unfortunately, the tombs discovered so far have been plundered by looters long ago. Beyond here, at the eastern end of the site, a paved road, the Calzada Caragra, runs for 200m before disappearing into thick jungle; the main entrance to town, this was believed to have once stretched for 20km.

The people of Guayabo brought stones to the site from a great distance, probably from the banks of the Río Reventazón, and petroglyphs have been found on 53 of these – most are now in the Museo Nacional in San José, but you can still see carvings of what appear to be lizard and jaguar gods, and an altogether more intriguingly patterned rock, the so-called Sky Stone, believed by some experts to represent a celestial map of the southern skies, and therefore possibly of use as an ancient calendar.

Other than this, little is known of the people who lived here, and there are no clues as to why Guayabo was ultimately abandoned, though hypotheses include an epidemic or war with neighbouring tribes.