A plunging wave occurs when the ocean floor is steep or has sudden depth changes, such as from a reef or sandbar. The crest of the wave becomes much steeper than a spilling wave, becomes vertical, then curls over and drops onto the trough of the wave, releasing most of its energy at once in a relatively violent impact. A plunging wave breaks with more energy than a significantly larger spilling wave. The wave can trap and compress the air under the lip, which creates the `crashing` sound associated with waves. With large waves, this crash can be felt by beachgoers on land. Offshore wind conditions can make plungers more likely. If a plunging wave is not parallel to the beach or the ocean floor, the section of the wave which reaches shallow water will break first, and the breaking section or curl will move laterally across the face of the wave as the wave continues. This is the `tube` that is so highly sought after by surfers also called a `barrel`, a `pit`, and `the greenroom`, among other terms. The surfer tries to stay near or under the crashing lip, often trying to stay as `deep` in the tube as possible while still being able to shoot forward and exit the barrel before it closes. A plunging wave that is parallel to the beach can break along its whole length at once, rendering it unrideable and dangerous. Surfers refer to these waves as `closed out`.
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