Emissarium of Lake Nemi, Ancient canal system in Genzano di Roma, Italy.

The Emissarium of Lake Nemi is an artificial conduit about 1,653 metres long, cut through the volcanic crater to link the lake with the Ariccia crater. Its outlet portal, of 18th-century date, opens a few metres above the water on the south-western shore, where a stretch of about 25 metres is lined with large opus quadratum blocks and a perforated stone slab served as a filter.

The conduit was cut in the 4th century BC by the Aricini, the inhabitants of Aricia, not by the Romans; the temple of Diana Aricina, built around 500 BC on ground the lake would otherwise flood, suggests a Latin work, possibly with Etruscan specialists. Two blind headings were driven without intermediate shafts, meeting 1300 m from the lake with errors of about 2 m horizontally and 3 m vertically.

The conduit was the work of the Aricini, not the Romans, probably with Etruscan specialists; the temple of Diana Aricina nearby would have been flooded without it. At the entrance, a slab with circular holes filtered branches and debris, and grooves in the stone held sluice boards to control the flow and clean the filter.

The conduit is not open for walking through; the entrance sits a few metres above the lake's present level on the south-western shore, half-hidden under a large fig tree, behind an 18th-century portal. From there the first 25 metres are lined with large opus quadratum blocks, with a perforated slab that served as a filter and grooves for sluice boards. Look for these details from outside rather than planning an interior visit.

The conduit was dug as two blind headings without intermediate shafts, starting from both ends. The team from the lake cut straight through soft tuff and pozzolana, while the downstream team hit hard basaltic rock after a few hundred metres; the headings met 1300 metres from the lake, with a horizontal error of about 2 metres and a vertical one of 3 metres.