Knock Up Stream

The Knock Up Stream is a powerful pressurized water stream that shoots upwards to the sky from the Blue Sea. This event only happens 5 times a month and the strength of each occurrence varies dramatically.

The Knock Up Stream is created by a giant underwater cave filled with gas. At some point in time, the gas pressure is too big and creates a crack in the ceiling, then a giant whirlpool is created from the crack. Once most of the gas gets free, the seabed explodes, sending a giant stream upwards.

Appearance
So far, the Knock Up Stream has made three appearances:
 * 1) When a part of Jaya was shot up to the White-White Sea near Angel Island.
 * 2) When the Roger Pirates used it to get into Skypiea.
 * 3) When the Straw Hat Pirates used it to travel to the White Sea.

The Knock Up Stream appears non-canonically in One Piece: Stampede, though it is slightly different. The Stream is shown pushing up a 'Treasure Island' in the central lake of Delta Island. In the movie, the Stream has a current which spirals upwards, allowing multiple ships to sail on it.

Trivia

 * Similar natural events do occur in nature like the Knock Up Stream and it is likely the event was based on a combination of these natural events. The likenesses include examples such as limnic eruptions, which are the result of a sudden release of carbon dioxide gas from the depth of deep lakes that have failed to cycle their water and geysers, which are fountain-like gushing streams of water caused by extreme heat and pressure.
 * In Greek mythology, the monster Charybdis can be compared the Knock Up Stream. It swallows sailors in whirlpools and shoots them into the air in the same manner as the Knock Up Stream.
 * The Knock Up Stream is not the only method to travel up to Skypiea, however, it is the only "all or nothing" method. Either the entire crew survives, or none survive.
 * According to Gan Fall in Chapter 238, as a result of its gamble, most crews traveling to the Sky Island take more favorable gamble routes to the island in which some members die while others survive.