When conditions are at their peak — in April, visibility can reach 100 feet underwater — stare down into the blue to see 90 percent of this massive iceberg from the surface. The structures of the giant in southeastern Greenland shimmer blue-greenish in the strong sunlight. The colossus reveals unexpected forms, similar to a reef with canyons, coves and small plateaus, which extend far beyond the structures of the iceberg that can be seen over the water’s surface. In some places, sharp edges like oversize axes rise from the ice, and in others, the ice is traversed by fine cracks that run like veins through it. Almost everywhere is a thin, transparent layer of ice over a solid white core that looks like snow.