A viewer asked me if I knew the purpose of this hole in the ground, a "snow well", and I knew, but cannot prove it now he has given away te secret. In the guide I read it was indicated the hole could store downpoor from the sky which would, evaporating, keep the timber in a sound condition.
The viewer has a slightly different version: this area was filled by snow brought from the high mountains for keeping the Mosque cool enough so that the prayers feel comfortable in hot weather.
I have the feeling he may be wrong in the sense that elsewhere on the internet one can find references to the snow just falling in through the hole in the roof that has been closed as late as 1965, by glass. That would imply the snow just found its way in by itself.