More than 600 swimmers from 13 countries have stroked their way through one of Russia's most scenic and historic settings, circling the St Petersburg island that holds the Peter and Paul Fortress.
The 2.3-kilometre circuit which some swam twice today took them around the iconic site noted for the needle-thin golden church spire that rises 122 metres high.
The cathedral is where most Russian czars since Peter the Great are buried.
It is a tough swim not only for its length, but because of strong currents and seaweed in the Neva River.
Yelena Vodolazova, 46, of St Petersburg says "it was not only a physical challenge for oneself but also an unforgettable experience of swimming in such beautiful historical location where swimming is normally restricted.
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