The Collaborative International Dictionary
Swallet \Swal"let\, n. [Cf. G. schwall a sea swell, from schwellen to swell, E. swell.] Water breaking in upon the miners at their work; -- so called among tin miners. [Prov. Eng.]
Wiktionary
n. 1 (context British English) A sinkhole; a shakehole. 2 (context UK dialect dated mining tin mining English) Water breaking in upon the miners at their work.
Usage examples of "swallet".
Beyond that the track to the swallet which Taffy and Jerry took, which Hall explored, turned off across the moor.
Carlswark Cave in the Peak District and Eastwater Swallet in the Mendips.
The water debouched into the passage where Modesty and Willie stood, spreading to fill a shallow stream-bed at their feet as it crossed the passage diagonally, to vanish into a swallet some distance from them on the far side.
You see, all the hills are honeycombed with swallets - I mean sort of natural pits that go down to streams flowing deep underground.