It might seem like common sense not to dig a big, unsupported hole at the beach and then go inside it, but sand has some really interesting geotechnical properties that can provide a false sense of security. When you build a sand castle, you know, the dry sand falls apart, and the wet sand holds together. In the US, OSHA has a detailed set of guidelines on how to stay safe when working at the bottom of a hole, including how steep slopes can be depending on the types of soil, and the devices used to shore up an excavation to keep it from collapsing while people are inside.