The Early History of the Channel Tunnel

heck, sometimes you encounter this in a domestic situation (e.g. Hawaii and the mainland US). It keeps diseases contained and is also meant to prevent animal trafficking.

https://www.gutenberg.org/files/66685/66685-h/66685-h.htm

Geological conditions existing in the middle of the Strait were, up to that time, almost entirely a matter of surmise, based on observations made on the British and French sides of the Channel, and in the process of finding out more about them, [18]Thomé de Gamond decided to descend in person to the bottom of the Channel to collect geological specimens. In 1855, at the age of forty-eight, he had the hardihood to make a number of such descents, unencumbered by diving equipment, in the middle of the Strait. Naked except for wrappings that he wound about his head to keep in place pads of buttered lint he had plastered over his ears, to protect them from high water pressure, he would plunge to the bottom of the Channel, weighted down by bags of flints and trailing a long safety line attached to his body, and a red distress line attached to his left arm, from a rowboat occupied also by a Channel pilot, a young assistant, and his own daughter, who went along to keep watch over him. On the deepest of these descents, at a point off Folkestone, Thomé de Gamond, having put a spoonful of olive oil into his mouth as a lubricant that would allow him to expel air from his lungs without permitting water at high pressure to force its way in, dived down weighted by four bags of flints weighing a total of 180 pounds. About his waist he wore a belt of ten inflated pig's bladders, which were to pull him rapidly to the surface after he had scooped up his geological specimen from the Channel bed and released his ballast, and, using this system, he actually touched bottom at a depth of between 99 and 108 feet. His ascent from this particular dive was not unremarkable, either; in an account of it, he wrote that just after he had left the bottom of the Channel with a sample of clay

... I was attacked by voracious fish, which seized me by the legs and arms. One of them bit me on the chin, and would at the same time have attacked my throat if it had not been preserved by a thick handkerchief.... I was fortunate enough not to open my mouth, and I reappeared on top of the water after being immersed [19]fifty-two seconds. My men saw one of the monsters which had assailed me, and which did not leave me until I had reached the surface. They were conger eels.