A sensor unit for use in sensing conditions in a pipeline comprises an untethered ball-shaped sensor unit adapted to roll along the interior surface of a pipeline, and an instrument package within the ball-shaped sensor unit. The package contains at least one magnetometer or accelerometer. Preferably, three magnetometers, arranged orthogonally, are present. Other sensors can also be present as required, such as an acoustic sensor to detect leaks and a temperature or chemical sensor. Recording means record the data acquired by the magnetometer(s) or accelerometer and the sensors, and optionally also record a timing trace.