Two French innovators, Jacques Cousteau and engineer Emile Gagnan, co-invented the Aqua-Lung, the underwater breathing gear that made scuba suits possible (via Live Science). However, Leonardo da Vinci actually came up with his own "diving suit" of sorts back in the early 16th century — one that, had it been built, would have looked like a perfect match for the many unknown horrors under the ocean.
According to Atlas Obscura, da Vinci may have conceptualized his rather nightmarish diving suit — which would have included goggles, a leather suit, a full face mask with two hollow breathing tubes, inflatable wine bladders for oxygen and buoyancy, and even a pouch for urine — to support the Venetians' efforts in fighting against the Ottoman Empire's army. Reportedly, da Vinci incorporated his extensive observations about tidal movements into his many diving suit designs. His scuba gear plans were apparently never realized, though, owing to the fact that the Venetians succeeded in fighting back against their invaders.
Similar to his other concepts, da Vinci kept certain details about the suit designs a mystery, to keep them from falling into the wrong hands: "This I do not publish or divulge on account of the evil nature of men who would practice assassinations at the bottom of the seas, by breaking the ships in their lowest parts and sinking them together with the crews who are in them."
