Euphrates Tunnel

The Euphrates Tunnel was a legendary tunnel purportedly built under the river Euphrates to connect the two halves of the city of Babylon in Mesopotamia.
The first tunnel known definitively to have been built underwater was the Thames Tunnel, completed in 1841.

History

A description of the tunnel as being built and used by Queen Semiramis is given by Diodorus in the Bibliotheca Historica:
Philostratus also describes the tunnel's construction in the Life of Apollonius of Tyana:

Construction allegedly began with a temporary dam across the Euphrates, and proceeded using a "cut and cover" technique.
The tunnel allegedly spanned 12 feet high and 15 feet wide. It is presumed that it was used by pedestrians and horse driven chariots and connected a major temple with the royal palace on the other shore of the river. It was supposedly lined with brick and waterproofed with asphalt.