The spelunker achievement is obtained by visiting the very bottom of any world map. There is no master version of this achievement.
Make sure you're on the very bottom of the world in order to receive this achievement. You should be standing on the bedrock (the invulnerable, darker earth block that lines the bottom).
Some worlds have teleporters or pre-dug tunnels to the bottom, which makes it one of the earliest achievements that new players can quickly get.
Once you've reached the bottom, you can simply type /sp to respawn back at the surface.