From Bulbapedia, the community-driven Pokémon encyclopedia.
A hole is a special tile in various games that allows the player to fall into another floor of caves.
In Pokémon Red, Blue, and Yellow, holes are found in the Seafoam Islands and on Victory Road. In the Seafoam Islands, the player has to push boulders down holes, and then follow them by going down the hole themselves. This eventually leads the player to Articuno. (When falling down the holes that lead to water, Surf is used automatically.)
In Pokémon Gold, Silver, and Crystal and their Generation IV remakes, there are holes that exist in the Ice Path. It is required to push boulders down these holes, which allows the player to go through different floors and eventually exit the cave.
In Pokémon Ruby, Sapphire, and Emerald, a variation of a hole appears: a cracked tile. These cracked tiles appear at the Sky Pillar, Mirage Tower, and Granite Cave. Cracked tiles can be cycled over as long as the player has enough momentum, but once cycled over they turn into regular holes and drop the player through. Cracked tiles reset when the player leaves the room that they are in. Stopping, slowing, walking, or hitting an obstacle while riding on a cracked tile will cause the player to fall through the hole.
There are also holes of a different kind that appear in Mt. Pyre. The player can walk into these holes and fall to the floor below the current floor. These holes give access to the Lax and Sea Incenses.
In Pokémon FireRed and LeafGreen, holes are in the same location as in the earlier Kanto region games, in the Seafoam Islands and on Victory Road. In the Seafoam Islands, the player has to push boulders down holes, and then follow them by going down the hole themselves. The boulders dropped down these holes block off currents, making certain areas accessible. This eventually leads the player to Articuno.
Furthermore, the cracked ice tiles in the Sootopolis Gym and Icefall Cave turn into holes after they have been stepped on twice.
In Unova, there is another type of hole, slightly reminiscent of the holes in Pokémon FireRed and LeafGreen. Rocks can be pushed into the holes with Strength so that the player can walk over the rocks. However, the player cannot walk into these holes. Once the rock has been pushed into the hole, it will stay there even if the player leaves and re-enters the area.
There are also special sand pits in Relic Castle that can trap the player if the sides of the pit are run across, and the sliding sand drags them to the center, where a hole awaits. Walking across the sides of the pit allows the player to cross them successfully. The sand pit hole leads down to the next level of the ruins.
The same type of hole that appears in Unova also appears in Kalos. Pushing the nearby rocks into those holes either allows access to an optional area, or provides a shortcut through the area.