A cluster of fishing boats is the source of the bluish light near the center of the image. The fisherman are likely luring Todarodes pacificus—a species known as the Japanese flying squid—to the surface with bright xenon bulbs. The city lights on the Korean side of the strait tend to have an orange glow, while those on the Japanese side are greener. The difference is related to the distribution of mercury vapor, metal halide, and high-pressure sodium lamps—the bulb types most often used for street and outdoor lightning. Mercury vapor lights tend to be green, high-pressure sodium is orange, and metal-halide lamps are bright white.