Six square miles of obsidian, 100 feet deep:
earthobservatory.nasa.gov

Obsidian—sharp-edged, translucent, and lustrous—is one of the most distinctive volcanic rocks. Its unique glassy properties result from a disordered structure: the atoms are irregular, like a liquid. Whereas crystalline materials like ice, diamonds, or granite have atoms arranged in repeating patterns, obsidian and other types of glass retain an irregular pattern because the atoms are frozen in place as the material solidifies. This can happen if a liquid is cooled very quickly (quenched in water, for example), or if it is so thick that the atoms have trouble moving through the fluid. Therefore extremely viscous, slow-moving lavas sometimes harden into obsidian.

It’s an amazing planet.