To the surprise of the Mongols, the town had no fortifications whatsoever. Its citizens didn’t even intend to defend themselves and were engaged in fervent praying, asking God for their salvation. On seeing this, the Mongols rushed to the attack, but then stopped. Suddenly, they saw countless fountains of water bursting from under the ground all around them. The attackers fell back and watched the town submerge into the lake. The last thing they saw was a glaring dome of a cathedral with a cross on top of it. Soon only waves remained.
Source: Kitezh – Wikipedia