One Piece world is a hollow world with two layer of landmass (upper layer and lower layer). The upper layer world is balanced on the Red Line (Red Line be the pillar that supports the upper layer world). The world is a small ball inside a large ball with both center are fixed in same point. The larger ball is held by Red Line that connects two ball on the center axis.

This can also explain why Enies Lobby and Lulusia hole is not filling. If the World government starts using Mother Flame frequently, you can expect the world to sink or the world its self starts to sink as the Red Line is not able to maintain the pressure of the upper layer. This may also verify due to frequent earthquakes the upper layer maybe collapsing inside. This can also explains the formation Red Line as if you give any outlet to pressure it will come side through that.

The Red Line may have formed like if you compress two connected half sphere the connected edges sphere expands outwards. This also explains why Red Line is high as this.

Great flooding: Take the world as two layer and there are holes on the upper layer and water in between layers. If we push the upper layer to the inside. The water in between will come outside and landmass will sink.

Knock up stream: If you see the image on Jaya arc and consider the hollow world is not full of water, compared the vastness of upper layer rocked the hole is small. So hollow earth will not be full of water like shown in the image. Imagine if you push plate with holes from the top on a container with water and air. First air comes out. Which is knock up stream

This also explains why the knock up stream is getting bigger. Jaya is near the oceans of Ennies Lobby and so the phenomenon is starting there. It also explains Ennies Lobby gate of justice as it like small sink holes on the upper layer where water drains down. I am talking about the whirlpool ocean current between three gates of justice. You can thing that is due to Mother Flame on the ocean floor of upper layer.

The Red Line is similar to red wall limestone on grandcanyon which was formed due to dead sea shells. My theory was somehow the middle layer of earth was stripped to form red line. The one piece earth may had large deposits of limestones on its middle layer.