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.

Burgas, sometimes transliterated as Bourgas, is the second largest city on the Bulgarian Black Sea Coast in the region of Northern Thrace and the fourth-largest city in Bulgaria after Sofia, Plovdiv, and Varna, with a population of 203,000 inhabitants, while 277,922 live in its urban area. It is the capital of Burgas Province and an important industrial, transport, cultural and tourist centre.