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Only 5% of our oceans have been explored, so clearly there's a lot we don't know about what goes on in the deep.

Sometimes discoveries are made that are later found to be entirely wrong.

The Sansha Yongle Blue Hole in the South China Sea was thought for years to be the deepest in our ocean floor.

But scientists have now discovered that the Taam Ja' Blue Hole off the coast of Mexico and Belize is actually the deepest - and far deeper than anyone previously thought.

Not only that, but the scientists now also believe that Taam Ja' may be part of a huge network of underwater caves and tunnels.

What is a blue hole?

Blue holes are underwater sinkholes.

A sinkhole occurs when rock is eroded by water and then collapses, leaving a big hole behind.

They're particularly interesting when they happen on the ocean floor, as they become home to lots of underwater life.

Race to the bottom

Taam Ja' Blue Hole is off the coast of Mexico's Yucatan Peninsula - close to where the asteroid that killed the dinosaurs struck 66 million years ago.

It was thought to be 275m below sea level when it was discovered in 2021.

This made it the second deepest blue hole on the planet.

But in December 2023, a team of Mexican researchers went on a scuba diving mission to take a second look.

They've now published the results of that expedition in a science journal called Frontiers in Marine Science, and have said it's actually more than 420m below sea level.

But that's not all - the equipment they used to measure it didn't actually reach the bottom, so it could potentially be a lot deeper than that.

As well as measuring how far down it went, the scientists were interested in the temperature of, and the chemicals found in the water inside the hole.

They found that after around 400 metres, the water in Taam Ja' started to resemble water found in the Caribbean Sea, which led the team to believe that there could be a network of tunnels deep within the hole that links it to other parts of the ocean.