Lycus

The Lycus river, also known as Lycos or Lykos, was a stretch of water that ran partially underground through the city of Constantinople.

It flowed southeast across Constantinople's main peninsula, cutting a straight valley between the first six hills to the east and the large Seventh Hill to the west. The Lycus is notable for the fact that the final two thousand meters of the river flow underground, before dumping into the Harbor of Theodosius on the southern edge of the city.