Sourdough bread keeps cracking underneath. What could be the reason? We have been facing this problem as you can see in the image. This dough is 75% hydrated and it is always cooked in a ventilated oven over a baking stone, the temperature of baking is 220° Celsius / 428° Fahrenheit, while it bakes we create steam inside the oven by sprinkling water inside it. 
In the beginning of the baking process the oven spring happens accordingly, after a while the oven spring stops upwards and the bread beggins to crack underneath, creating in some cases huge cracks. What can we do to prevent it? What could we having being doing wrong? Thanks in advance
Cracks around the bottom are said to be caused by cold baking sheets. Something about the temperature gradient needs to change. Try baking on a preheated pizza stone.