Sinkholes Are a Land Phenomenon, Not an Ocean One

Sinkholes do not form in the open ocean. They are a karst landform created on land when slightly acidic groundwater dissolves soluble bedrock (limestone, dolomite, gypsum, salt) over thousands of years, creating underground voids that eventually collapse.

Why Not in the Ocean?

• Mechanism differs: Ocean floors are shaped by plate tectonics—seafloor spreading at mid-ocean ridges, subduction at trenches, volcanism, and sediment deposition—not by groundwater dissolution of bedrock.
• Water chemistry: Seawater is already saturated with calcium carbonate; it doesn't aggressively dissolve limestone like CO₂-charged rainwater does on land.
• Pressure: Deep ocean pressure suppresses void formation and collapse dynamics.

What Does Exist in the Ocean

• Blue Holes: Vertical marine caves (e.g., Great Blue Hole, Belize) formed during past low sea levels when they were dry caves; now submerged.
• Submarine Canyons: Carved by turbidity currents, not dissolution.
• Pockmarks: Seafloor depressions from gas/fluid venting.
• Collapse Structures: Rare, associated with salt diapir dissolution or magma chamber withdrawal, but not "sinkholes" in the terrestrial sense.

So if you see a hole in the ocean, it's not a sinkhole—it's a blue hole, a vent, or a tectonic feature. True sinkholes are strictly terrestrial.