Use in the 16th Century
The cavern must have been open to the Irish Sea back in the 16th century, because according to one of Salty's tales, Captain Calles literally "sailed" into the cavern. The cavern was used by the pirate to hide his pirate ship when the merchant ships and the navy tried to track him down. Captain Calles never retrieved his ship; yet there was also the possibility that the navy did not call off the search until Captain Calles' death. The cove entrance collapsed at one point, causing the pirate ship to stay there undisturbed for over 400 years.
During the construction of the Harwick Branch Line, the cavern caused the track bed to crumble and sink, forcing the workmen to divert the track. Later, when Thomas started helping with the construction, he did not notice the danger signs and went right over the collapsing ground. The ground caved in to a deep, dark underground tunnel, and Thomas fell truck after truck until eventually terminating in the cavern where the pirate ship stood. Rocky was called to lift Thomas back up to the tracks, and also lifted the pirate ship, to everyone's amazement. Two nights later, Thomas met the pirate Sailor John and his railboat Skiff, who had been searching for the entrance into that cavern for years. Thomas lowered John in, and unbelievably found the treasure map which should have blown overboard. However, whatever remained in the cavern was destroyed. Some sparks caused by bad coal from Ryan's funnel ignited some trucks of dynamite, and Thomas shunted them into the cavern in a series of quick manoeuvres. The crates in the first truck fell into the cavern and exploded, with the cavern seemingly collapsing.
The cavern is located between Arlesburgh West and Harwick. It could also be considered as a giant sinkhole. It was once open to the Irish Sea, and the tunnel leading down into it from the rails was long, deep and dark. The cavern made the track bed above it collapse, forcing the branch to divert away from it. It is unknown what happened to it after Thomas shunted the dynamite in, but there could be two possibilities: one is that the caved-in entrance that opens to the Irish Sea was blasted open again by the force, and the other is the whole cavern collapsed after the explosion.