Only about 20% of the ocean’s depths has been mapped by humans. Here’s what we do — and don’t — know about the deep seas and why studying them is so risky.

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

A history of ocean exploration. The first submarine was built by Dutch engineer Cornelis Drebbel in 1620, but it stuck to shallow waters. It would take nearly 300 years — in the aftermath of the ...

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

It would take nearly 300 years — in the aftermath of the Titanic disaster — before sonar technology began to offer scientists a clearer picture of what lies in the ocean's depths.A major step ...

We're not far from where the expedition launched on Sunday. It was an hour and 45 minutes into the dive that the vessel lost contact with its mother ship. It's been lost and silent since then. Now ...

We're not far from where the expedition launched on Sunday. It was an hour and 45 minutes into the dive that the vessel lost contact with its mother ship. It's been lost and silent since then. Now ...