Stolobny Island is an island on Lake Seliger in the Tver Oblast of Russia, about 10 km (6.2 mi) north of the town of Ostashkov. The island is the home of Nilov Monastery, which was founded by Saint Nilus in 1594, and previously welcomed up to 40,000 pilgrims each year. Most of the buildings of the monastery were built in the 18th and 19th Centuries in a neoclassical style. Today the monastery complex remains one of the most impressive ensembles of Neoclassical architecture in Eastern Europe. Some of its churches date back to the 17th century. A graceful embankment was completed by 1812, and a large cathedral was built in 1821-25. The construction of the causeway to the island was completed in 1812. The Nilow Monastery was one of the largest and wealthiest monasteries in the Russian Empire.
In 1919, after the October Revolution, the monastery was confiscated. It was closed in 1927 by the Soviet government and subsequently used for various purposes. From 1927 to 1939 there was a work camp for underage criminals.
In the period 1939 to 1941, during the first years of World War II, the monastery was a prisoner of war camp of the Russian secret service NKVD, which held approximately 7,000 Polish prisoners of war who had been taken captive by the Soviet Union as a result of the Molotov-Ribbentrop Pact. Almost all of the prisoners were subsequently executed in April 1940 in Kalinin (now Tver) and then buried in mass graves in Mednoye, an act which became known as the Katyn Massacre. Amongst those killed were Polish officers, lawyers, policemen, teachers, doctors, and other members of the intelligentsia.
From 1941 to 1945 was in the building complex to a hospital, and again from 1945 to 1960 a camp for minors and orphans. 1960 to 1971 the monastery was used as a retirement home, and from 1971 to 1990 a hostel for tourists. After 1990, the complex was given back of the Russian Orthodox Church, and in 1995 it opened again as a functioning monastery, which it still is today.