This story originally appeared on time.com
On June 30th, 1908, an asteroid entered the Earth’s atmosphere above Tunguska, Siberia and exploded with the energy of about 185 Hiroshima-level atomic bombs. In memory of that catastrophic event, every year on June 30th is Asteroid Day, a “global awareness campaign where people from around the world come together to learn about asteroids and what we can do to protect our planet, families, communities, and future generations from future asteroid impacts.”
Rocks from space are constantly bombarding the Earth. While most burn up completely in the atmosphere, some larger ones are able to reach the Earth’s surface. When they do, they leave behind an impact crater. Most impact craters are eventually erased due to erosion, vegetation growth, and the movement of the Earth’s tectonic plates. (On the pesky matter of terminology: a rock in space is an asteroid; a rock in space that enters our atmosphere is a meteor; a rock in space that enters our atmosphere andreaches the ground is a meteorite.)
In the rare case that an impact crater is preserved, water can fill the depression, creating a lake. When the lake is connected to the region’s water system, it is almost indistinguishable from any other lake, except to geologists who study the composition of the rocks, searching for ‘impactite‘, a type of melted rock that forms in a meteorite strike. In other cases, when the water that has collected is unconnected, impact crater lakes can provide a valuable geologic and fossil record by preserving ancient sediment.