On 10 July 2011, Bulgaria sank in the Kuybyshev Reservoir of the Volga River near Syukeyevo, Kamsko-Ustyinsky District, Tatarstan, Russia,[3] with 201[4] passengers and crew aboard[5] when sailing from the town of Bolgar to the regional capital, Kazan.

[7] The sinking of Bulgaria was Russia's worst maritime disaster since 1986, when the SS Admiral Nakhimov collided with a cargo ship and 423 people died.

Investigators claim that Argorechtur was operating the cruise ship without a proper licence, and the company director was arrested on 12 July 2011.

On 10 July 2011, Bulgaria was traveling in Tatarstan on the Volga River when she was caught in a storm and sank at about 13:58 Moscow time (09:58 UTC), several hours after beginning her cruise.

This was apparently compounded by the captain trying to turn the boat around, and soon water rushed into the vessel through portholes that had been opened because the ship had no air conditioning.

[9] On 11 July 2011, a government official from the Ministry of Emergency Situations said that the likelihood of finding additional survivors was slim, leaving a presumed total of up to 129 dead.

According to survivors' accounts, two ships (the oil tanker Volgoneft-104[17] [other sources claim it could have been the Volgoneft-38] and the freighter Arbat) passed by after the Bulgaria had sunk.

In accordance with Russian criminal code article 270,[18] the captain of a ship that refuses to help in disaster could be sentenced to up to two years of deprivation of freedom.

However, these ships, being heavy freight barges with minimal crews, were not technically capable of stopping while passing, nor of turning back in an acceptable time.

On 11 July, an anonymous source close to the committee investigating the sinking said that the likely cause was portholes that were opened because of the lack of air conditioning on the vessel, which allowed water to enter Bulgaria when the captain attempted to turn the ship during stormy weather.

Investigators also suggested that the boat set sail with a list to the right, possibly due to full sewage or fuel tanks on that side, and with one of its engines not properly functioning.