Rumbling and roaring, the sea around the Scottish island of Orkney is a frightening beast.
It destroyed hundreds of ships and killed many sailors, until one man revealed what lay beneath the waves.
In the 18th century, the waters around Orkney were busy with ships crossing the North Atlantic Ocean for trade.
Orkney's coastline was uncharted.
Hazards like rocks and sandbars lay hidden underneath the waves, causing many shipwrecks.
In 1743, local schoolmaster Murdoch Mackenzie decided to change this.
Sailing out in his boat, he used a rope and a lead weight to measure the depth and formation of the ocean floor.
From these measurements, he mapped the coastal waters to warn sailors of the hidden dangers.
In doing so, McKenzie invented new mapping symbols.
Sandy Firth, Sailor, UK - "McKenzie was the first man to start putting in these symbols. Now they indicate the nature of the bottom of the sea. He gives you the state of the tide at different times, and the direction of it..."
These symbols are still used today.
But the technology used to survey the seas has advanced.
Today, ships like this coastguard boat are specially equipped to map out the waters.
Right now, they are investigating an accident.
A ship was massively damaged next to these small islands due to shallow waters.
Mackenzie's chart, which is still in use, tells us that here, the sea should be 26m deep.
This is deep enough for any ship to pass.
But over the 260 years since McKenzie made his map, new ways of finding submerged hazards have been invented.
The coastguard's boat emits sound waves into the water beneath, and detects their reverberations.
The quicker the sound waves bounce back, the shallower the water.
The depth can be measured several thousand times every second, giving a very accurate reading.
The coastguard can now create an image of the seabed.
The image shows sharp rocks over 10m high, which lower the depth of the water.
Now, it is only 7.5m deep at its shallowest point, rather than 26.
This makes it impassable for most ships.
The coastguard's survey puts a new hazard on the map.
Hundreds of years since the first coastal maps, the dangers of the world's oceans continue to be uncovered, and lives continue to be saved.