Regular

The tunnel was better built here than the ones nearer the well. And there, at the end of it, was a pale light, and Angua, crouched by one of the big, round dwarf doors. Sally paid her no attention. She barely glanced at the dwarf slumped with his back against the bottom of the door.
Instead, she stared at the symbol scrawled large on the metal. It was big and crude, and might be a round, staring eye with a tail, and it gleamed with the greeny-white glow of vurms.

– a mine sign |
Terry Pratchett, Thud!