Biri Island, about an hour northeast of Catarman by jeepney to Lavezares and a ferry crossing, contains some of the most dramatic coastal geology in the Philippines. The Pacific Ocean has spent millennia carving the island's basaltic shoreline into formations with names like Magasang, Bel-at, Abay, and Cagbalo-an — each a different expression of what happens when equatorial sea meets ancient volcanic rock.
The most accessible route is a bangka tour of the island's perimeter, typically arranged through guides at the Lavezares pier. The boat pulls close to formations that cannot be reached on foot, and the tour usually covers the main formation sequence in three to four hours. You will need to wade and scramble at several points; leave valuables in a dry bag. Accommodation on Biri itself is minimal — most visitors make it a day trip from Catarman.