tube

Hollow interior formed as a wave crest arcs over and down to the trough. Tubes are generally formed at breaks where the incoming wave moves suddenly over a shallow area—a sandbar or a high spot in a reef or point. A land-to-sea offshore wind will help create a tube, as it lifts and supports the crest as it pitches forward. Most waves don't tube; of those that do, few are shaped properly to allow t...