I once spent a week exploring the Yucatan Peninsula of Mexico. The seaside ruins of Tulum, an ancient Mayan city, stand atop a cliff that juts out into the ocean. A local guide told me it was possible to swim under the cliffs from one side and come out the other. To accomplish the feat, I swam along underwater passageways in the direction of the opposite side. The sun provided light reflecting against the sandy ocean floor.
Along the way, breathing was made possible by swimming up against the rocky ceiling wherever an air bubble could be found and inhaling the air. The bubbles form when tidal action washes aerated water under the cliffs. Micro bubbles in the surge concentrate into one large bubble until they resemble big glass dinner plates clinging to the submerged rock ceiling. They were plentiful at first, along with good visibility.