One or other of the two whirlpool pools are marked on most old maps of the area (though rarely both, save on the 1775 Map) as Whirlpools or "Whirly Pools". This Map shows their relationship to the major features in the 1880s, and here's the modern Map (see also these 1800s Tithe Maps: Map 1; Map 2). The pools were actually resurgencies (ie. where water that has gone underground elsewhere wells up). As water disappeared from the Troggy into the limestone of the area at Llanmellin, it is felt that they were probably resurgencies of this water, though it is not clear whether any kind of proof of this was ever gained¹. As mentioned, there have been two major reorganisations of the Troggy: the first was the diversion at some point c.1600 CE or before of the stream below Deepweir from Sudbrook to Caldicot Pill. The second, and more important here, was the major concreting of the whole catchment as part of the building of the Severn tunnel.

The building of the Severn tunnel went largely without incident until 18th October 1879, when the tunnellers on the Welsh side hit a large freshwater-filled fissure which flooded the tunnel to the roof: at its maximum discharging some 1,200,000 gallons per hour (5681.25 m³h^-1)². After this "Severn Tunnel Great Spring" opened, engineer Thomas A. Walker (Photo: Biography) was contracted to solve the problem*. Years of extremely dangerous and inventive diving saw the spring plugged in the tunnel*, however, to reduce the amount of water entering it, the chief engineer decided to seal the places the water was entering the geology. As the Spring was under Portskewett, this meant concreting the floor of the Troggy and any holes along its length, possibly including the whirlpools³.

This may seem weird (why cap somewhere where the water is leaving the ground? Indeed, why call somewhere where water is coming out a whirlpool?) however, the strange nature of the whirlpools may explain this.