I n 2010, the audio producer Roman Mars launched 99% Invisible, a podcast about the hidden designs and inventions most of us overlook. At the time, he didn’t have high hopes for it. Not only was the subject matter wilfully niche – early topics included building acoustics and the ergonomics of the toothbrush – but the episodes were just four minutes long. Still, listener numbers quickly grew and the episodes got longer. Sixteen years on, 99% Invisible is now a podcasting institution that has amassed more than 660 episodes, investigating everything from political logos to the colour of margarine. “It’s taking something that seems really boring and going, ‘No, no, no, this is interesting,’ and really convincing you of that,” Mars says. Mars, 51, is talking over video call from a small, foam-panelled studio at his home in Berkeley, northern California, where he records the show.…