This well-received BBC series on technology, and its application in the ancient world through several practical experiments, continues tonight with the carving of Olmec Heads. More than 3,000 years ago, in the fertile jungles of lowland Mexico, the first civilisation in the Americas was born. Five hundred years before Rome was founded, the Olmec were building great cities with pyramids and ball courts. Considered by most archeologists to be the 'mother culture' of Mexico, the roots of the later Maya and Aztec cultures lie with Olmec. And one of the most extraordinary and spectacular feats of the ancient Olmec was their monumental sculptures. The inhabitants of the site of La Venta erected huge sculptures, some of which weighed up to 40 tonnes. The most famous of these are the massive stone heads. Believed to be portraits of their leaders, they are more than two metres high and sculpted entirely without the help of metal tools. There are thousands of tonnes of stones at the site but, most extraordinary of all, these sculptures are found in an area where there are no rocks. The area identified as the source of the Olmec boulders lies in the mountains more than 160 kilometres away. And between these mountains and La Venta the land is criss-crossed with massive rivers and swamps, which would have made the transportation even more difficult - though evidently not impossible. So how did the stones get to La Venta?