The Zapotec civilization flourished in southwest Mexico from about 200 BC up till the Spanish invasion in the 16th Century. The earliest known remains are those of an already relatively advanced culture in terms of its cities and agrarian infrastructure. In art ,architecture, mathematics and primitive science, the Zapotecs have a clear affinity with other civilizations to the south such as the Mayans and Olmec, yet their history contains no records or hints of their lineage.
The Zapotec capital was Monte Alban which lies at the top of an artificially leveled mountain promontory, south of the present-day city of Oaxaca. Systematic excavations in the early 1930s revealed gold, jade, precious gems and crystals, and turquoise in several of the tombs. Also discovered was a network of stone-lined tunnels, too small for use by normal people , either adults or children. The excavators� initial guess that they had discovered a drainage system, investigation however proved this theory implausible.
The first of these tunnels was 20 inches high and 25 inches wide - 195 feet into the tunnel they came upon a skeleton, an incense burner, and funeral artifacts jade, turquoise, pearls and stones. A few yards beyond this the tunnel was blocked, and the explorers had to dig a 25-foot shaft from the surface beyond the blockage, to reenter it
Further along, they found even smaller passages, no more than a foot high, running off from the main tunnel. Leading into one of these was a tiny set of steps. 320 feet from the main entrance, they found another skeleton, and a few yards beyond this, at the edge of the northern terrace of the great plaza, the tunnel ended.
2 Similar tunnels packed with clay were also later found. Slightly to the east of one of the wealthiest tombs another complex of miniature tunnels was discovered, all meticulously lined with stone, some were less than a foot high.
Smoke was blown into this tunnel complex which revealed 'a number of unexpected exits.'
The excavators initial guess that it was a drainage system was abandoned. Nor were the the tunnels a network of escape routes ,as normal size Humans would have to crawl on their backs or bellies to navigate them. It is quite possible they used in some sort of burial rituals, an elaborate tomb system similar to the catacombs of Europe -considering some skeletons were found within, but officially their function remains a mystery.
David Hatcher Childress, in Lost Cities of North & Central America