Swirl marks are a network of microscopic scratches in the top layer of the clear coat. These are not circular in reality, but they appear circular because they refract light from a single point (like the sun or a streetlamp). This damage is caused by mechanical abrasion, typically from using dirty sponges, "automatic" brush car washes, or dry-wiping a dusty surface. When dirt particles are trapped between a wash media and the paint, they act as an abrasive, "carving" tiny V-shaped grooves into the resin. This scatters light instead of reflecting it, making the paint look dull, grey, and hazy.