Lake Mandrensko or Lake Mandra is the southernmost of the Burgas Lakes, located in the immediate proximity of the Black Sea and close to Burgas. Being 8 km long and 1.3 km wide at maximum, as well as having an area of 38.84 km², it was a brackish natural lake until 1963, when it was turned into a reservoir with the construction of a dam to secure fresh water for the large Neftochim Burgas oil refinery. The rivers Izvorska reka, Fakiyska reka, Sredetska reka and Rusokastrenska reka flow into the lake.

Develtos or Deultum was an ancient city and bishopric in Thrace. It was located at the mouth of the river Sredetska reka on the west coast of Lake Mandrensko, previously part of the Gulf of Burgas, and near the modern village of Debelt.