Editing Lake Nemi (section)

===The emissary===
[[File:Ingresso attuale emissario.jpg|thumb|140px|Entrance to the emissary]]
[[File:Emissary plan.jpg|thumb|300px|Emissary plan]]
[[File:Entrance emissary.jpg|thumb|Map of entrance]]
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Lake Nemi, like that of Albano, has no natural outlets and all the water comes from rain in the crater. Originally the level must have greatly fluctuated. Hence the lake level was controlled from rising too high in ancient times by an [[Emissary (hydraulics)|emissary]] connected to a 1650 m-long tunnel dug through the crater wall and ending in Vallericca c. 10 m lower. It dates from before circa 300 BC when the upper inlet was closed and replaced by an entrance around 11 m lower to further reduce the level of the lake and which had a series of chambers.<ref>P. BILDE The Roman villa by Lake Nemi: from nature to culture – between private and public, Proceedings of a conference at the Swedish Institute in Rome September 17–18, 2004 p 7</ref>

In planning the tunnel the shortest path from the lake to the outside was used consistent with the minimum downhill slope needed. The tunnel connects the lake with the adjacent crater of Ariccia, which was also occupied by a lake at that time and later reclaimed.

It was a monumental, if invisible, project which involved many workers not least to dispose of the excavated material and to transport it away. The original tunnel varied between 0.7 and 1 m wide meaning that only one person could be at the excavation face at a time. No intermediate shafts were used in the central part, unlike other tunnels, to speed up the work and lower the orientation error, but to set the initial direction of the tunnels at the two ends several vertical shafts were included there. Tunnelling would have started at the outlet end to avoid flooding of the workface from groundwater but when hard basalt rock was encountered tunnelling also from the inlet seems to have been added.<ref>Emissario di nemi http://www.lambertoferriricchi.it/test2/wp-content/uploads/2015/01/pdf/NEMI.pdf</ref>

The emissary entrance is a few metres above the current level of the lake, on the south-western shore. An 18th century portal gives access to a first stretch of tunnel clad with large opus quadratum blocks for about 25 m. A stone plate with circular holes, part of which is still in situ, was a filter to prevent entry of logs and other large debris. Grooves in the walls are clearly visible for bulkheads which could control the flow of water and allowing the filters to be cleaned. The original plan was for the entrance to be on the same level as the floor of the main tunnel but there is a steep slope from the outside to avoid water entering the tunnel while under construction. The entrance floor was to be lowered once completed but this was never done.

From the entrance a narrow tunnel leads, after about 150 m, into the trapezoidal tunnel.

After Vallericca the water flowed in an open-air channel for about 2 km before another underground section for 600 m, the so-called Aricino tunnel.