You’ve probably read or heard a lot about how prices keep going up for gas, food, clothes, electronics, and pretty much everything else. That’s called inflation, and it’s a problem for nearly everyone. But here’s a story about deflation—and it’s a problem only if you live in Great Britain and your parents, or maybe even you, drive a big S.U.V.

Since March, a group of anonymous activists has been prowling the streets of Britain’s cities at night, dressed in dark clothes and avoiding streetlights and security cameras. Their mission: letting the air out of the tires of Range Rovers, Mercedes G-Classes, BMW X5s, Audi Q8s, and other tank-like, gas-guzzling, road-hogging S.U.V.’s.