Cave Expert's ice pile can refer to two of the five Ice Piles in the game.
The first is located right outside the entrance of Ice Crystal Cave, and like every Ice Pile, expands and collects anything sold to the Cave Expert right next to it, naturally spawning an Ice King once it reaches critical mass.
The second is located inside the Ice Crystal Arena deep with Ice Crystal Cave. This Ice Pile requires more Ice than normal Piles, upon reaching critical spawns a hideous Cave Creature.
Both Piles are only accessible once paying the Cave Expert the requisite fee for mining in Ice Crystal Cave - 20,000 Ice and 75,000 Money