Neither the name of the first mine that fell in, nor the date of its collapse, is recorded. We know that a mine in Witton fell in in 1750, and another to the north of the Northwich Town Bridge followed in 1759, and that many others collapsed before 1770. Lakes, or “flashes” as they are called locally, have formed over the larger of these sinkings, but the sites are more commonly marked by what are known as rock-pit holes, and large tracts of country are scored with these funnel-shaped indentations. There can be no doubt that a number of these old mines were worked with pillars that were too few and slender for the purpose, and these supports gradually weakened to their ultimate collapse under the pressure of the superincumbent earths. As the sinking did not take place evenly all over the mine, but most frequently occurred near the shafts and at the greatest distance from the sides of the cavity, the roof would curve down towards the sinking centre and the falling-in formed the V-shaped apertures on the surface which are described as rock-pit holes. But, while in a percentage of cases the collapse of the mine could be traced to the crashing of the pillars, the destruction of the majority of the mines was caused directly by the influx of water, although this water, having become saturated with salt, would, if undisturbed, cause no further havoc in the interior of the mine. But the manufacture of white salt from brine, which was temporarily surpassed in importance by the rock-salt industry, was not discontinued, and a copious supply of brine flowing over the rock head of the upper bed, was tapped by shafts and pumped to the surface. When, about 1850, this supply showed signs of failing, attention was directed to the enormous reservoirs of brine in the old inundated mines, into which had drained a great quantity of the rock-head brine. The attempt to pump brine out of the abandoned workings was successful, and for some years an abundant supply was obtained. After 1870 the pumping operations caused further collapses, the land overlying the mines subsided, and lakes were formed which, at intervals, broke into the partially exhausted reservoirs, and, pouring through the top-rock workings into the mines in the bottom bed of salt, replenished the supply of brine. A great collapse which occurred in the Dunkirk district in 1880 let down the waters of Cranage Brook and the Wadebrook, together with a huge quantity from the river Weaver. The subsidence resulted in the formation of a large lake, which, following upon a later subsidence in the same area, suddenly disappeared into the earth and literally flooded all the underlying strata.