The Solovki prison camp (later Solovki prison), located on the Solovetsky Islands in the White Sea, was a large forced labor camp for political prisoners of Imperial Russia, and later the Soviet Union. During the Soviet era, as "Solovki Special Purpose Camp" it became a model where the NKVD developed and tested security measures, "living conditions", work production norms for prisoners, and different methods of repression. The exact number of prisoners who went through the camp during 1923–1939 is still unknown, but estimates range between tens and hundreds of thousands. It was the "mother of the GULAG" according to Aleksandr Solzhenitsyn.