Some of the earlier sinkings of this kind, when pumps had to be depended on for keeping down the water, were conducted at great cost, as, for instance, at South Hetton, and more recently Ryhope, near Sunderland, through the magnesian limestone of Durham.
This method has been adopted in various sinkings on the continent of Europe.
The laying out of a colliery, after the coal has been won, by sinkings or levels, may be accomplished in various ways, according to the nature of the coal, its thickness and dip, and the extent of ground to be worked.
Jebel Akhdar, being without "faults," has no deep internal valleys, and presents, the appearance of downs: but its seaward face is very deeply eroded, and deep circular sinkings (swallow-holes) are common.
These interior landslips, besides choking the pipes and breaking the communication, often produce sinkings at the surface.
The immediate cavities or pits into which the tracheal stigmata open appear to be in many cases ectodermic in sinkings, but there seems to be no reason (based on embryological observation) for regarding the tracheae as an ingrowth of the ectoderm.
No tracheate Crustacea are known, but some terrestrial Isopoda develop pulmonary in-sinkings of the integument.