Undetected gas kicks are at the root of many disastrous accidents in the oil industry. A major factor in these tragic outcomes is the suddenness with which oil and gas can be ejected from the top of the riser, which gives operators virtually no time for an adequate response. This aspect of the phenomenon is quantified by providing a physical explanation of its origin and suggesting simple, if approximate, guidelines to estimate its severity. Then a relatively straightforward and potentially practical way to detect the presence of dangerous gas volumes in a riser is described. The basic idea is to measure the pressure difference between sensors spaced along the riser. When gas occupies the space between sensors, the pressure difference undergoes large changes chiefly caused by the decreased hydrostatic head. It is shown that this difference can be enhanced by adequate signal processing and is robust in the presence of noise. The suggested detection method is supported by a set of laboratory experiments.