There is a vault that covers the casket in the ground and the space between the vault and the ground around the vault is not very big. When the grave is filled in the dirt gets trapped at the top of the vault and does not fill in around the vault. Over time the dirt will settle in around the vault which causes the ground to sink at the surface. The cemetery personal review the graves and will top off the dirt periodically until the settling stops. Depending on the weather this can take up to 18 months. Once the settling has stopped the grave is compacted and top soil and seed applied.