Estrov was the Russian town where Yassen Gregorovitch was born and grew up in. The town only appeared in the tenth novel Russian Roulette.
Russian Roulette[]
Due to a chemical accident in town, (caused by Vladimir Sharkovsky's carelessness and greed,) the then young Yasha (future Yassen) had to escape to a nearby town before he was hunted by the Russian government, leaving his family, who wanted the accident to be known by nobody, especially after the Chernobyl accident.
All of Yassen's family members died in the accident, as well as his best friend Leo, leaving Estrov wiped off the map. Estrov is believed to be 255 miles slightly southeast of Moscow.