In Minecraft, what material must you mine to access the Nether world?

Obsidian

Which prophet foretold the birth of John the Baptist? Cuba celebrates Christmas after removing a decades-long ban in what year? Which band formed by siblings Caleb, Jared, and Nathan Followill is known for "Use Somebody"? Who was the first NFL running back to rush for 100 touchdowns in their career? Who first described conic sections analytically?