The Hurtle Pot is a pot (hole in the ground) at Chapel-le-Dale. There follows an atmospheric late nineteenth-century description:

Here is another dismal hole overshadowed by impending rocks, beneath whose shelving recesses a deep and murky pool pulsates with a hollow glutting sound, and to which an entry can be obtained by a narrow opening on the southern side, trending downwards with a steep and noisome slope. The air is heavy with the repugnant odours of the broad-leaved allium – ramsons – rendered more pungent by the vapours exhaling from the filthy bottom, or the cavernous passages wherein they have been pent. The depth of the pool is twenty-three feet, the height of the rock fifty-eight above the water, and the distance from Chapel-le-Dale Church about seventy yards. After a heavy downfall of rain the glutting sound, to which allusion has already been made, is most marked. This intermittent throbbing sound, issuing whence the surface of the water and the hidden ledges meet, is called the Fairy Churn, or Hurtle Pot Boggart, and may at other times be much increased by throwing a heavy stone into the ebon depths (Baldeston 37)

Here is an earlier reference that shows that the boggart was mobile!

At the bottom [of Hurtle Pot] is a dark pool, very deep, in which trout are sometimes caught; I should suppose they must be of the kind said to frequent the great cave of Kentucky,– eyeless, for organs of vision can be of little service in such a region. This pool or ‘pot’ is of a much more gloomy character than the first, and, not unnaturally, the good folks of the neighbourhood associate with it a boggart that now and then appears to frighten people from their propriety. Of the adventures of the said boggart, and the pranks it plays, the inhabitants of the dale tell many a tale; some of these stories bear a strong family likeness to the tales of other boggarts in other parts of the world. (Dobson, I, 22.)

The earliest reference we have been able to find to the boggart comes from 1823:

A curious phenomenon occurs in this cavern, caused by the glutting of water against the surface of rocks, after heavy rains. A singular noise is heard to proceed from the surface of the water, which the country folks call ‘The Hurtlepot Boggart,’ or the ‘Fairy Churn.’ (Otley 199)

However, note that earlier descriptions hint at the horror of the place!