There are several places in the National Park System where you can find bombs. One of them is Craters of the Moon National Monument and Preserve in Idaho.
But the bombs aren't your typical explosives. Rather, they were assembled during volcanic eruptions thousands of years ago. Here's a primer on the types of bombs you can find at Craters of the Moon:
Four kinds of volcanic bombs are found at Craters of the Moon, all of which began as a volume of molten rock that is ejected into the air. If the lava gets twisted during its flight, it is called a spindle bomb and typically measures from a few inches to several feet in length. If it is very tiny and twisted, it is called a ribbon bomb. When the volume of lava forms a crust that is cracked by expanding gases as it flies through the air, it is called a breadcrust bomb, which exhibits a surface texture that resembles bread rising in the oven. If the lava mass does not completely solidify during flight, so that it flattens and spreads on landing, it is called a cow-pie bomb. Some cow-pie bombs are over 10 feet long.
This week's mystery photo was of the breadcrust variety. Definitely not something you're going to slice with a bread knife, though.