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 bot 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 will happen only five times a month.

Appearance
So far, the Knock Up Stream has made 2 appearances:
 * When a part of Jaya was shot towards Skypiea.
 * When the Straw Hat Pirates used it to travel to Skypiea.
 * 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.

Trivia

 * Similair natural events do occur in nature like the Knock-up Stream and it is likely the event was based on a combition of these natural events. The likinesses 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 through the air. In the same manner as Knock Up Stream.