Is it permissible to bury the dead in places other than graves, such as digging a large underground pit that can accommodate a number of them, placing stones inside it like shelves for the dead to be laid upon, and then sealing it with a large stone when it is full?

The basic principle is that each deceased person should be buried in a separate grave, and no one else should be buried with them unless there is a need or necessity, such as limited space or a large number of deceased. In such a case, the one who recited the most Qur'an among them should be placed closest to the Qibla, and a barrier of earth should be placed between every two deceased. The Sunnah is to dig a grave for the deceased in the ground and then make a niche (lahd) for them. However, building underground tombs in the form of houses where the deceased are placed, or arranging the deceased on shelves, is contrary to the Sunnah.