We haven't talked about ancient engineering for a while now, having been up to our knees in molasses and mal-functioning toilets, so this month I'd like to take you back to every schoolchild's favourite piece of engineering - the secret tomb. If you haven't ever imagined yourself as a later-day Indiana Jones, dashing down concealed corridors and dodging ancient booby-traps to discover a priceless treasure then you really should, not least because there is actually a grain of truth to the movie legend.

By 900 AD the city of Palenque in what is today Southern Mexico was a ruin, covered in forest and slipping from memory. It would be over 650 years before any outsider would see the remains and another 250 years of occasional reports before the site came to the attention of archaeologists. Following the publication of Descriptions of the Ruins of an Ancient City discovered near Palenque in 1822 interest in these mysterious Central American lost cities began to grow in the West and in 1839 Frederick Catherwood, an architect and draughtsman, and John Lloyd Stephens, a diplomat and writer finally travelled to the region where they glimpsed a pyramid poking through the thick undergrowth of the Chiapas foothills. Setting up camp in the ruins Stephens and Catherwood went about recording what at first appeared to be an isolated temple but which they soon realised was an entire city choked by almost impenetrable rainforest.

The temple they were camping in is known now as the Temple of the Inscriptions and had been begun in around 675 AD when Palenque was the capital of possibly the largest and most important kingdom in the Mayan world. A 23 metre high stepped pyramid with a temple structure on the top, the building was decorated with the second longest inscription known from the Mayan world. Not that Stephens or Catherwood could read the text. Instead they simply drew what they saw - a beautiful, empty temple in a long abandoned city with no sign or clue to where the people had gone, what fate had befallen them or where they were buried.

Had they been able to read it they would have learned that the inscription records 180 years of Mayan history at the city and in particular details the life and achievements of the man who commissioned the temple, K'inich Janaab' Pakal, who is known today as Pacal the Great - the man who had revived the city's fortunes. What they never guessed was that he was still there. Nor had they any reason to. Since the rediscovery of Mayan civilisation it has been widely assumed that the stepped pyramids found in their cities were simply elaborate supports for the temples on their summits. There was no evidence for tombs, no evidence really for any of the people who must have once lived in these places. Just empty, ruined stone buildings.

And this was still the received wisdom when Mexican archaeologist Alberto Ruz Lhuillier visited the site in 1949. However, standing where Stephens and Catherwood had stood 110 years earlier he noticed a double row of stone plugs set into one of the slabs on the sanctuary floor which he guessed marked the lifting points where the stone had been lowered into position implying there might be something beneath it.

Removing the stone he found he was right. Beneath lay a vaulted passage so full of rubble that it took four digging seasons to clear and it was 1952 before his team arrived at the bottom. Here they found their way blocked by a wall next to which stood a stone box which contained pottery jars, shells filled with the red pigment cinnabar (mercury sulfide), beads, jade earplugs and a solitary pearl. Removing the wall blocking their path they next came to a more grisly discovery: - a chamber containing the skeletons of six human sacrifices, beyond which lay another large block of stone.

On the other side lay something unheard of in Mayan archaeology - a royal tomb in the heart of a pyramid. Inside a large slab covered an elaborately decorated sarcophagus, standing on six short piers, beneath which lay pottery food dishes and two life-size stucco human heads. The lid of the sarcophagus was decorated with one of the most important scenes on any Mayan monument, a depiction of its owner, Pacal the Great himself who is shown falling into the netherworld whose jaws gape open to receive him.

Beneath the lid lay the body of Pacal, sprinkled with mercury sulfide and surrounded by over 700 jade items. He wore a jade diadem, a net skirt made up of jade pieces held together with gold wire, necklaces, pectoral decorations, rings, bracelets and ear-flares. Over his face lay a mosaic jade mask and at his feet rested two jade statuettes. In his hands he held a jade cube and sphere whose significance is still a mystery.

It was the find of the decade and one of the greatest ever Mayan discoveries. Here at last was one of the inhabitants of these lost cities, and one of their greatest - hidden for nearly 1300 years, surrounded by his treasures. It was the sort of discovery you only find in movies and even today, with perhaps only 5 percent of this 65 square kilometre city excavated, Pacal's kingdom holds on to many more secrets.