Built in 1910, the mosque's enormous blue-tiled roof was modeled after the striking complex of tiled buildings in Samarkand, Uzbekistan, housing the mausoleum of medieval Muslim conqueror and thinker Tamerlane. The mosque was a gift to St. Petersburg from the emir of the Silk Road city of Bukhara, then part of the Russian empire. Closed down by the Bolsheviks, it was used as a medical equipment warehouse during World War II. It was the largest mosque in the Soviet Muslim world -- even larger than the Uzbek ones it emulated -- and is still a major symbol for Russia's 20 million-plus Muslims. Its gray granite walls contrast vividly with the sky-blue mosaics covering the dome and minarets. Inside, columns of green marble break up the vast prayer spaces.