Mil Tracta (original Sapkowski's Elder Speech name: Mil Trachta), formerly known as Centloch, is a lake-dotted region north of Metinna near the Amell Mountains. Tarn Mira is one of the many lakes and the legendary Tor Zireael rises from that lake.
"'The land of a Hundred Lakes, once called Centloch,' Vysogota explained, 'is called Mil Tracta today. It's a very vast lake land bisected by the River Yelena in the northern part of Metinna, close to the border with Nazair and Mag Turga." - pg. 334, The Tower of the Swallow (UK edition)