Regular

Although they were flickering a little, the lights were also coming back on in Vimes’s head. He peered into what was now no longer blackness but merely gloom, and gloom was like daylight after the darkness that had gone before.
“…got to read to Young Sam…” he whispered, to a cavern of giant stalactites and stalagmites, all gleaming with water, “…to read to Young Sam…”

– the mission | Terry Pratchett, Thud!