Varicose veins are enlarged and twisted veins, often manifesting as dark blue or purple in color, near the surface of the skin and usually seen on the legs and ankles. Varicose veins may appear enlarged and protrude from the surface of the skin. Some even display a worm-like appearance just beneath the skin. Weakened valves are the cause of varicose veins and they typically develop over time. Normally, the one-way valves in a vein keep blood flowing up the leg and back toward the heart. When those valves don’t work as well as they should, blood pools in your legs and pressure builds up.