A description of the tunnel as being built and used by Queen Semiramis is given by Diodorus (fl. 50 BCE) in the Bibliotheca Historica:
"After all these in a low ground in Babylon, she sunk a place for a pond, four-square, erery square being three hundred furlongs in length, lined with brick, and cemented with brimstone, and the whole five-and-thirty feet in depth: into this having first turned the river, she then made a passage in form of a vault, from one palace to another, whose arches were built of firm and strong brick, and plaistered all over on both sides with bitumen, four cubits thick. The walls of this vault were twenty bricks in thickness, and twelve feet high, beside and above the arches; and the breadth was fifteen feet. This piece of work being finished in two hundred and sixty days, the river was turned into its antient channel again, so that the river flowing over the whole work, Semiramis could go from one palace to the other, without passing over the river. She made likewise two brazen gates at either end of the vault, which continued to the time of the Persian empire."
— Diodorus, Bibliotheca Historica, Book II,1 (Translation by G. Booth, 1814).