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 little more than rocky protrusions from the ground, and must be unblocked by directing Pikmin towards them. 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.