The Grimpen Mire was a vast bog, deep in the heart of Dartmoor in Devon. It was, in local lore, connected to the legend of the hellhound which terrorised the Baskerville family in "The Hound of the Baskervilles".

The mire was located in an especially isolated part of the region, and there were few nearby settlements. The largest was the hamlet of Grimpen, seat of the parish of the same name, where Dr James Mortimer had his practice. There were also several scattered and isolated farms and houses located around its periphery, including Baskerville Hall, Sir Charles Baskerville's ancestral house; Merripit House, residence of Jack and Beryl Stapleton; Lafter Hall, home to Mr Frankland; and the farms of High Tor and Foulmire. The nearest town was Coombe Tracey. Dartmoor Prison was located on the edge of the mire, fourteen miles from Grimpen.

The mire was a misty and wild place. Watson describes it as "melancholy". It was surrounded by many large rock formations, among them Black, Vixen, and Belliver Tors. There were numerous Neolithic archaeological sites, including the ruins of ancient dwellings and barrows. However, it was also quite dangerous due to the bog at its center. Stapleton tells John Watson that "a false step yonder means death to man or beast", and recounts how he saw a stray pony get sucked into the bog. It appeared that only Stapleton was able to cross the Mire safely; however, it was discovered he had marked where to step on the mire with sticks but discouraged Watson going there as he may discover Stapleton's plans. The Grimpen Mire was also known to be where the Hound was kenneled. Upon inspection of the Mire, phosphoros is found  which gave the hound its terrifying appearance. At the end of the case, Stapleton himself is believed to have died after trying to cross the bog. This is seen as a logical surmise as the footsteps of Stapleton stopped suddenly.

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