This wide valley is "basically a big drain, thirty-miles of soft limestone rock, edged by mountains of harder rock" — the Ramtops. The soft valley floor is constantly eroded by the rapidly flowing Koom River and by hundreds of encircling waterfalls. Water thunders down into the valley from a height of half-a-mile from "The Tears of the King" falls. The valley is riddled with sinkholes, pits, and caves; the landscape is changing constantly as waters flow above ground and below, making boulders shift and uprooting whole trees. Tourists often get lost, sustain injuries, and even die when hiking the valley. After the events of Thud! the Valley has become a tourist attraction; however, it is still a dangerously unstable place to walk.
After visiting Koom Valley and hearing voices, the maddened artist Methodia Rascal created a 50-foot panascopic illustration of the valley, showing in detail the Battle of Koom Valley, with annotations in the margins. The Ramkins eventually donated the painting to the Royal Art Museum (see Thud!). When Sybil Ramkin was a schoolgirl she created a scaled-down replica of the painting.
Koom Valley seems to be another redundancy joke, combe or coombe (Welsh: cwm) being an old word for a valley of that shape. See also the Forest of Skund.
It may also convey connotations of Doom Valley.