Koom Valley
Koom Valley is located far north of Ankh-Morpork in the near Ramtops. It is the site of the Battle of Koom Valley and several more battles over the centuries, always between dwarfs and trolls.
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.
A map of the valley was sketched by hiker Eric Wheelbrace in his book Walking in the Koom Valley.
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.
Lady Sybil's school friend Bunty Waynesbury lives in Ham-on-Koom on the outskirts of Koom Valley with her husband, the local magistrate.
Annotations
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.