How do you check that a petroleum well is leak-proof, so that it cannot endanger the environment or the platform staff? To do so, you’d have to investigate a narrow hole with a diameter of maybe 30 cm, kilometers below the ground, where the temperatures can reach well above 100°C and the pressure is crushing. This may sound difficult, and it is! Even so, the petroleum industry has been doing things like these for almost a century, and over the past four decades they have increasingly been using ultrasonic techniques.