It seemed like nothing at the start, but looks like it’s much bigger than just Asakusa. A huddle of young customers at your 7-Eleven, buying bulk boxes of chocolate bars. You can’t fail to notice it goes on every night for months. They crowd out the cash register. What on earth could be so popular? It doesn’t help to tune into their conversation, because they’re speaking Chinese. So when your curiosity wins out, you just ask, in English. Jennifer and Cindy? We are tourists staying around here, says Cindy. We fly out tomorrow, and you know, you can’t get Black Thunder so cheaply in Taiwan, and this Big Thunder not until April.

Mystery solved. You hear later the tourists even strip the shelves of all the Black Thunder, Big Thunder, Morning Thunder and Chibi Thunder from the huge Don Quijote store down the street. Not to mention the White Black Thunder, which you’ve yet to see. A bit more digging and you find these super cheap, tiny-size chocolate bars launched by a regional Japanese confectioner 20 years ago went little noticed for a decade until word-of-mouth and some clever promotion sparked interest that built into a sort of cult following, with Black Thunder Wikipedia pages, even one in English. Just 30-yen gets you a beefy sugar hit wrapped in rice bubbles, cookie and chocolate. They taste all right – in fact you’ve bought the original Black Thunder for years – even if the mouthfeel from eating a few too many is a little like motor oil.