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Our oceans are full of mysteries and places yet to be explored - and they cover over 70% of our planet.

Many scientists say more is known about the surface of mars than is known about the bottom of our oceans.

Under the surface lie massive mountains, deep valleys and sink holes - these giant voids are known as 'blue holes'.

One such hole is the 'Green Banana' in the Gulf of Mexico, off Florida's coast, and a team of scientists are getting ready to head into it.

So what do we know about blue holes and what secrets are scientists hoping to uncover?

Blue holes are submerged springs and sink holes, most of them form in limestone rock, which can be very porous - meaning it has lots of tiny holes in it.

Over time, perhaps centuries, water flow erodes the limestone and creates cracks, crevasses and caves leaving a void or an opening.

Blue holes that can be seen from the air are in shallower water making them easier to spot, but the Green Banana is extremely deep.

It was discovered in the early 1990s, but we now know it is not alone in the Gulf.

Sitting about 20 miles off Florida's coast are 20 verified blue holes.

Jim Culter, an expert diver studying bottom-dwelling marine life for Marine Laboratory Mote, in the US, has been mapping them out.

"There are at least 20 and maybe two to three times that many, and maybe many more," he told Newsround.

The first site Jim dived into was the Amberjack Hole, named after the fish that have made the water around it their home.

"It's very exciting... you go down to the edge and it's like a reef, where you have many fish, except the rock and corals are not above the bottom, they go down into the actual bottom, like a reef in reverse."

"As you go down further the diversity of animals and plants decline, and then everything starts to get darker and colder... it gets to be kind of surreal because it's so different." Jim added.

Jim is now working with a team of scientists, funded by National Oceanic and Atmospheric Administration (NOAA), to find out more about what is going on within the holes.

The team's next dive site is the Green Banana, a hole first discovered by a fisherman.

It is over 30 metres (100ft) across and is 129 metres (425ft) deep, but nobody knows how big it is at the bottom.

Dr. Emily Hall is the Principal Investigator on the project, she says the holes are brimming with marine life and present a number of questions that the team want answers to.

Emily told Newsround: "If you're swimming further away from the holes, it's pretty barren - there's not a whole lot out there on the the bottom of the Gulf, but when you get closer you see soft corals and sea grasses, and then when you get to the hole it just explodes with life, it's really amazing."

The team are heading into the holes to take long-term measurements, to find out what is going on inside and outside of it, biologically and chemically.

Emily asks: "What's going on out there, will they tell us anything about the future?

"Are these holes unique or are they connected to each other genetically? Do fish that are around this hole leave there and go to another hole? Or are they nurseries for certain fish or other marine organisms?"

"The PH goes down quickly as you go in, which is what we are expecting to see with ocean acidification and climate change. So we are interested to see what's going on in these holes that might tell us about the future of our oceans." she adds.

The mission to head into the abyss will take place in August.

As well as divers collecting samples, the team hope to lower a huge lander, shaped like a triangular prism into the hole to also collect samples.

But, this will be no easy task as it weighs around 600lbs.

Several blue holes have already been found and explored, although there may be many more.

The deepest hole to be discovered is the 'Dragon Hole' in the South China Sea, it was found and explored in 2016 and is thought to be 300m deep.

The team exploring it looked at tidal water levels and temperature when they carried out their research.

The Great Blue Hole in Belize is a spectacular sight from the air.

This hole is 318m wide, 124m deep and is home to many threatened species, including marine turtles and manatees.

It is a popular spot with divers, but in 2019 two submarines headed deep into the hole on an expedition to capture new images and footage for the first 3D map of its interior.

Dean's Blue Hole is 202m deep and is located in a protected cove on Long Island in the Bahamas.

In 2016 New Zealand's William Trubridge set a new world record for the deepest unassisted freedive, making it 102m down into the hole.

The 36-year-old held his breath for four minutes 14 seconds as he descended into the depths.