The mud snail is the world’s fastest snail! With help from an air bubble, it can reach the surface of the sea and drift with the current instead of slithering along, enabling it to reach a speed of up to 4 km/h. Mud snails range in colour from dark yellow to brown and are only 7 millimetres across. At first they’re very hard to see. But once you’ve found one, it won’t be the only one: up to 20,000 mud snails can inhabit a square metre.