1) An underground stream 
place where water disappears underground
• (n.) Water breaking in upon the miners at their work; -- so called among tin miners.
Usually related to karst, may be considered as a form of sinkhole, but could refer to a streamsink; (often associated with a cave entrance) and is one of the major entry points for recharge waters that drain underground in carbonate rock areas such as limestone. Swallets may empty directly into open or choked cave features such as shafts or avens, ...
Swal'let noun [ Confer German schwall a sea swell, from schwellen to swell, English swell .] Water breaking in upon the miners at their work; -- so called among tin miners. [ Prov. Eng.] 
an underground stream. · an opening through which a stream descends underground.