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Shatter cones are distinctive cone or fan-shaped features in rocks, with radiating fracture lines that resemble a horsetail. They are found in only two places on Earth, 1) in nuclear test sites and 2) meteorite impact structures. They are formed as a result of the high pressure, high velocity shock wave produced by a large impacting object or a large explosion. They range in size from less than 1 centimeter to more than 5 meters across and indicate that the original rock was shattered — somewhat like a car's windshield hit by a stone.
The exact formation mechanism of shatter cones is still not well understood. Besides large asteroid or comet impacts, only nuclear tests generate enough heat and pressure to even come close to the same process. The first man-made shatter cone was produced in 1959 during an underground nuclear explosion. But, as long as you know that you're not in a nuclear test site and you find a shatter cone in a rock, it is definitive evidence of a meteorite impact event. Shatter cones are the only shock indicators that can be seen with the naked eye. Others, such as planar deformation features (PDFs) can only be seen with the aid of a microscope.
Shatter cones can be found within the central uplifts of large impact structures and occasionally within the crater-fill deposits. At the Haughton impact structure in Canada, we find shatter cones in both of these settings: