Geyser

Pikmin 1
In the first game, small geysers can be found in various locations throughout each of the five areas except The Forest of Hope. Geysers are usually inactive, resembling fire geysers, and must be unblocked by directing Pikmin towards them. An exception to this rule is a geyser in a hole near the UV Lamp in the Distant Spring. Active geysers will bubble and send Olimar flying into the air when he stands on them, allowing access to otherwise unreachable high-up areas and sometimes providing shortcuts over large features.

Pikmin 2
Large geysers are found in every cave in Pikmin 2, always on the final sublevel. Their purpose is as a means of returning to the surface: Olimar, Louie, and the Pikmin jump into the water, which lifts them upwards through a hole in the ceiling and land at the landing site of the area the cave is found in. Some longer caves also have them on so-called "rest levels", generally a few floors down, as a means of leaving if you are lacking in Pikmin. Rest levels often also contain eggs and Candypop Buds.

Glitch
This is a visual glitch that occurs with geysers and cave entrances in Pikmin 2's Challenge Mode. These objects are sometimes visible underneath the surface of the cave, and can be seen by looking underneath the floor from somewhere else. These geysers are only visible underground in Challenge Mode because they start under the surface and rise up when The Key is collected.