Slime chunks
One chunk in ten is a slime chunk, picked by the world seed. Below Y 40 slimes spawn there at any light level, which makes them the place for a slime farm.
Get the seed with /seed in game. X and Z are where you stand (F3), or press F3 + C in game and paste into either box.
25 × 25 chunks around you · north is upoutlined: your chunk
Point at a chunk to see its blocks.