Knock Up Stream

The Knock Up Stream (突き上げる海流（ノックアップ ストリーム）) is a powerful pressurized water stream that shoots upwards to the sky from the bottom. The Knock Up Stream is created by a giant underwater cave filled with gas. At some point in time, the gas pressure is too much 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. This event happens only 5 times a month.

Appearance
So far, the Knock Up Stream has made two appearances: The blast is strong enough to send a ship and even one time an island straight to Skypiea. It first appeared in Chapter 236 and Episode 152.
 * When a part of Jaya was shot towards Skypiea.
 * When the Straw Hat Pirates used it to travel to Skypiea.

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 CO2 gases 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.