Nanatsugama is a line of sea caves and cliffs carved by the Genkai Sea north of central Karatsu. The basalt columns and dark openings are best when waves are active enough to show why the caves exist, but not so rough that boat trips stop running.
You can see the coast from land, though a boat gives the clearer sense of scale. It is a good counterpoint to Karatsu's softer castle-and-pine scenery.