This is Hermitage Pond, which is almost certainly the 'fishery' listed in the 1086 Domesday Survey 'close by to the mill'. Dividing the pond is the Midland Railway viaduct that replaced Portland Bridge in 1871 when the track at Kingsmill was 'smoothed' out allowing for faster trains.

Thomas Beck, Bishop of St David's held one piece of ground 'under the Hermitage ' of East Thwaite in Mansfield Moor in the King's Domain of the Forest. In 1302 the Pryor of Lenton was the Bishop's tenant here using it as a summer dwelling and sanatorium for sick monks. Hermitage means dwelling for hermits or monks. My guess is the original hermitage is under the house on the bank of the pond (middle) on the lane called The Hermitage.

The original Hermitage Viaduct built 1871 was constructed in wood but was later replaced in stone and packed with earth. (see photo above)
Here's the Midland Railway construction team with the diver and air pump.

Left, we have the Midland Railway engineer/diver submerging into the Hermitage Pond, 1871. Right, is the Hermitage Mill built in 1782 the first mill to be built on the Maun after the 'Arkwright revolution'. Richard Arkwright introduced the first water-wheel-powered, mechanised spinning frame in 1771 at Cromford Mill, reducing manufacturing costs and increasing production. Arkwright, a non-engineer but more entrepreneur had realised the potential for a mechanised spinning machine developed by a clock-maker whom he quickly employed to perfect whilst applying for a patent in 1769, which he gained in 1775 thus preventing anyone copying the idea. However, his huge success, wealth and ever increasing list of patents led to numerous bitter legal disputes on the grounds of him copying unpatented developments and by 1780 his grip on the industry was becoming weakened due to numerous patent infringements. Finally, in 1783 his patents were officially rejected in court. Meanwhile, Mansfield's cottage industry of frame knitters, totalling around 700 at the end of the 18th century and a major employer of the poor, became under an ever-growing threat from Arkwright's mills forcing the landowner, William Cavendish-Bentinck, the 3rd Duke of Portland to seek partners in the hosiery business that could compete with Arkwright and save the jobs of Mansfield. The Duke's land at the Hermitage was leased by Samuel Unwin Jr, the son of Samuel Unwin Sr, an established pioneer in the hosiery business from neighbouring Sutton in Asfield. Samuel Unwin Jr was co-financed by James Heygate a wealthy London banker, whom, was introduced to the enterprise through family marriage connections. By 1782 they had built the Hermitage Mill for cotton spinning. The mill still stands today, lately used by a building supplies company although I believe the site has now been vacated.