The Giant’s Causeway (Clochán na bhFómharach: the little stone pile of the Fomorians) off of northeast coast of Northern Ireland was formed many years ago by the giant Finn MacCool, as explained here. An alternate theory has it being formed sixty million years ago by lava cooling quickly (possibly by coming into contact with water) after a volcanic explosion. We may never know for sure which of those stories is true, but what is true is that a lot of the basalt rocks formed into hexagonal columns.
Photo by code poet; licensed under Creative Commons Attribution ShareAlike 2.0
I’d heard of these rocks before, but encountered them again in a photo by Lyn Miller under Found Math on MAA Online. Her photo was taken at Devils Postpile National Monument in California near Yosemite, which also has an impressive array of columns that average two feet in diameter.
Photo by Daniel Mayer; licensed under Creative Commons Attribution ShareAlike 1.0
Finally, if you google “hexagon lava” you find posts like this. It gives Lava Lamp a whole new meaning.