Secrets of the Ancient Olmecs

The ancient Olmecs in Mexico sculpted massive stone heads 3000 years ago, before even the Roman Empire was established. Thought to be carved in the likeness of their leaders, these heads weighed over 3 (up to 40) tonnes and came from a mountain over 160km away, a trek that would have taken them across rivers and swamps. How did they manage the task of transporting these huge blocks of rock? Without metal tools, how did they carve these dense stones?