Sewell’s Cave, also referred to as Cave S is located on the eastern side of the Rock of Gibraltar, near Holy Boy's Cave. Sewell Cave is named after Major J.W.S. Sewell, a Royal Engineer stationed in Gibraltar who was active in cave explorations during the early 20th century. Sewell also gave his name to another cave he discovered, Sewell's Fig Tree Caves, which are known today as Goat's Hair Twin Caves.
Sir Charles Warren had previously carried out a survey of Gibraltar in 1864 which recorded the cave as being 800 feet (240 m) above sea level and 30 feet (9.1 m) above the Great Gibraltar Sand Dune which later became the water catchments.
W.L.H. Duckworth who visited the cave with Major Sewell in 1910 found human remains which he considered were probably not of a modern man. In his notes Duckworth wrote:
It opens on the Mediterranean face of the Rock due east of St Michael’s platform and almost immediately beneath a higher cave, designated Holyboy’s Cave on a survey made by Sir Charles Warren. I refer to it as Cave S . . .
About 30 feet below Cave S, the great catchment area (constructed by the Admiralty) abuts on the nearly vertical rock and marks the upper limit of the underlying sand-slope now concealed by corrugated iron plates. The cave is accessible to skilled and active Cragsmen only but thanks to the kind co-operation of Mr. Wakeford of H.M. Dockyard, two of the men constantly engaged on the catchment area were placed at my disposal. These men fixed ropes and a ladder which made access to the cave perfectly easy.
He described the cave's floor as falling in height towards the entrance and it was covered in a soil that had the consistency of snuff in 1910. Despite not finding any bats he thought that the snuff-like material was vegetable matter and bat guano.
Duckworth later exhibited his various cave findings – including those from Cave S (Sewell’s Cave) in the Garrison Library before these were transferred to the Gibraltar Museum. These consisted of various human artefacts such as pottery, stone implements and other stone objects, a shell armlet, perforated cyprcea, charcoal, burnt bone and broken shellfish and a wide variety of bird remains and mammalian fauna have been found in the cave. The human bones were thought to come from one male skull-less skeleton which appeared to have notably thick bones.
Link to Neville Chipulina 2019 - The 200 Caves of Gibraltar – Part 3