<p>Literature reveals that rolling waves are the precursor to slug formation. The present research demonstrates how the rolling waves undergo plunging wave breaking before the slug formation. The wave-breaking phenomenon is visualized in the present experiments using a high-speed camera from the front and top. Flow visualization reveals different stages of wave breaking and the characteristics of the resulting slug formed. In addition, pressure signals are measured from the pipe wall during the wave breaking. It is observed from visualization that the air jet breaks the wave and plunges into the downstream liquid, creating a secondary vortex. This secondary vortex, aided by the Bernoulli effect at the wave crest, pulls more liquid towards the top, creating a liquid slug between two air packets. As the superficial gas Reynolds number increases, the air jet breaks waves multiple times before the slug forms. As a result, the slug formed is highly aerated. With an increase in superficial liquid Reynolds number, the number of times the jet breaks the wave before slug formation is reduced. So aeration in the slug is reduced. The pressure signals show spikes associated with wave breaking, and the recurrence plot from pressure signals illustrates the slug formation.</p>