What are in sinkholes the ocean?

Sinkholes are not found in the ocean. They are formed on land when the bedrock beneath the land surface is dissolved by groundwater, causing the land to collapse and form a depression. Sinkholes are most common in areas with soluble bedrock, such as limestone, dolomite, and gypsum. They can also be caused by human activities, such as groundwater pumping or mining.

In the open ocean, the ocean floor is formed by the constant movement of tectonic plates. When tectonic plates collide, the denser plate is forced beneath the less dense plate in a process called subduction. This process can cause the formation of deep ocean trenches, but not sinkholes.