|A fish captured during the Malaspina Expedition|
Credit: CSIC / JOAN COSTA.
What’s more, there are enormous numbers of these mesopelagic fish. In fact, there are so many that they create a sort of false sea floor:apparently, when sonar first began to be widely used during World War II, frustrated operators kept detecting what looked like solid ground at about 300 metres, even when they knew the ocean bed was a thousand or more metres deeper than that that. It proved to be an acoustic illusion created by the swim bladders of millions and millions of mesopelagic fish.