Those vivid blue lines across your hands are part of a delicate network just beneath the skin, and in many cases, they’re completely normal. Pale or thinning skin, low body fat, aging, athletic training, pregnancy, or even a hot day can make them stand out dramatically. For some, they’re simply a side effect of a strong heartbeat and good circulation. But when veins become twisted, rope-like, and raised, they can signal varicose changes and struggling vein valves, sometimes increasing the risk of blood clots that can travel and become life-threatening.

The real danger comes with change: veins that suddenly bulge, throb, feel warm, or sit under reddened, swollen skin should never be brushed off. Those could be early clues of deep vein thrombosis or other vascular issues. Staying active, avoiding long periods of sitting or standing, and protecting your circulation helps—but listening to your body, and seeing a doctor when something feels off, may be what saves your life.