The Layer to bottom command lowers the active
layer to the bottom of the layer stack. If the active layer is already
at the bottom of the stack or if there is only one layer, this menu
entry is insensitive and grayed out.
7.18.1. Activating the Command
You can access this command from the image menubar through
Layer → Stack → Layer to Bottom,
or by pressing the Shift key and clicking on the
down-arrow icon at the bottom of the Layers dialog.