To reach Hell (aka the Underworld) you need to dig 40 blockc deep or walk to 3,400 distance.

If you plan on digging you should walk at least 150 blocks from the spawn though as this will actually bring you a little closer, and without doing it there will otherwise be a massive drop down to hell that will kill you!

It is also a very hard biome and you will probably need diamond+ weapons and a lot of ammo. It is the only place where Undead Dragons spawn.

Hell is also the only place where you can mind Bloodstone!