Soak the rye berries in about 6 cups of the hot spring water, or at least enough to cover berries by 1-inch. Let stand 8 hours or overnight (if the water becomes completely absorbed before soaking time has ended, add more hot spring water to cover). Drain and reserve the soaking liquid. Add enough fresh room-temperature spring water to the reserved soaking liquid to measure 6 cups total. Reserve. Combine the starter, rye berries and reserved 6 cups water in a 6-quart bowl. Break up the starter well with a wooden spoon and stir until it loosens and the mixture is slightly frothy. Add 1 cup (5 1/2 ounces) of the rye flour and all the cracked rye. Stir well until combined. Add the salt and remaining flour. Stir until combined and the mixture is wet and sticky. Take the dough's temperature -- the ideal is 78 degrees. Cover with a clean damp towel and put in a moderately warm (74 to 80 degrees) draft-free place.
Note: If the dough temperature is higher than 78 degrees put it in a cooler than 78 degree place, like the refrigerator, until the dough cools to 78 degrees. If it is lower than 78 degrees, put it in a warmer than 78 degree place until the dough warms to 78 degrees. The point is to try to keep the dough at 78 degrees during its fermentation. If you have to move the dough, be gentle and don't jostle it, or the dough might deflate.
The dough will become spongy but not springy, distinctly sour-smelling and increased in volume by about 1/4. Generously grease 2 (5 by 9 by 3-inch) loaf pans with vegetable shortening and dust with rye flour. Turn the dough into the prepared pans. Smooth the tops with a wet thin flexible cake spatula. Cover the loaves with a clean damp towel or plastic wrap and put in a moderately warm (74 to 80 degree) draft free place until the dough has domed slightly and increased in volume again by 1/4. Bake on center rack of a preheated 300 degree oven until the loaves have shrunk from the sides of the pan and the tops are dark brown and a toothpick inserted in the center comes out clean, about 2 1/2 to 3 hours. Remove the loaves from the pans and hold the loaves upside down. Strike the bottoms firmly with your finger. If the sound is hollow, the breads are done. If it doesn't sound hollow, bake 15 minutes longer. Cool the pans for 10 minutes, then remove and cool completely on wire racks. Let the bread rest 24 to 36 hours before eating. It is very moist in the center when removed from oven, but as it cools the moisture distributes evenly throughout the bread. It will keep for weeks at room temperature wrapped tightly in plastic wrap.
Recipe courtesy of Dan Leader