Some blind people play basketball, mountain bike and skateboard with no technology at all, using sharp tongue clicks and the echoes that come back. This episode explains active human echolocation: why a palatal click works better than a hum, and how the returning echo tells the brain an object’s location, size and density, so that a wooden fence sounds duller than a metal one. In 1749 Denis Diderot described blind people sensing silent objects, and for centuries this was called facial vision, thought to be skin feeling air pressure, until experiments at Cornell in the 1940s showed that blocking the ears made the ability disappear. Donald Griffin coined the word echolocation in 1944.

A 2014 fMRI study by Lore Thaler recorded echoes inside expert echolocators’ ear canals and played them back in the scanner. In the experts the echoes lit up the primary visual cortex, while untrained sighted volunteers showed no visual activity. The episode then follows Daniel Kish, who lost both eyes to retinal cancer at 13 months and built the FlashSonar method, Ben Underwood, who rollerbladed and played street football by clicking, and Lucas Murray, a British boy taught by Kish after his parents saw Underwood on television.

• The precedence effect: sighted brains suppress echoes because the eyes already map the room
• Kish long resisted the white cane, then combined it with echolocation in what he calls perceptual mobility
• His charity World Access for the Blind takes blind teenagers mountain biking and hiking
• Underwood lost his eyes at three, taught himself to click by five, and died of cancer in 2009 as a teenager
• Kish taught Lucas Murray the basics in four days, and by seven Lucas could identify materials by sound