Dolphins 'see' through total darkness using sound bounced off their own jawbone

Dolphins fire rapid clicks from their nasal passages, focus the beam through the fatty 'melon' on their forehead, and pick up the returning echoes through fat-filled canals in their lower jaw. This biosonar is sharp enough to sense a fish's size and shape in water too murky or dark for eyesight to work at all.

— Wikipedia contributors, Animal echolocation