Alternatively, a fast black hole can leave a narrow tunnel in a solid object while passing through it. For example, a 10 22 g black hole should leave a tunnel with a radius of 0.1μm, which is large enough to be seen by an optical microscope. We could look for such micro-tunnels here on Earth in very old rocks, or even glass or other solid structures in very old buildings. While our estimate gives a very small probability of finding such tunnels, looking for them does not require expensive equipment and long preparation, and the payoff might be significant.