The 'Underwater City' on Online Maps Is Made of Data, Not Streets

The grid on the ocean floor is real, and it was made by people — but you could never swim down and touch it. The straight lines that look like the streets of a lost city are not on the sea floor at all. They are in the data: the tracks of ships that mapped one patch of seabed in finer detail than the area around it.

Grid lines on a map of the ocean bottom, northwest of the Canary Islands. Credit: NOAA.

The "city" people keep finding

Online mapping tools such as Google Earth have made exploring the planet easier than ever, and many of them now include detailed maps of the sea floor. With people all over the world browsing the seabed, many have come across patterns that look like grids — formations that seem too regular to be natural, and that lead people to ask whether they have found lost cities or underwater streets.

One of the most often cited is a spot in the Atlantic Ocean northwest of the Canary Islands, at 31°15'15" N, 24°15'30". The gridded section looks like the streets of a small town, but it is more than 100 miles wide.

How the sea floor gets mapped

Oceanographers map the bottom with sonar — sound. Ships typically tow submersible devices that send out sound waves; the sound travels through the water, bounces off the sea floor and returns to the ship. The echoes build a sound "image" of the peaks and valleys below.

NOAA is one of many agencies around the world surveying the sea floor this way, and their surveys differ. Some cover small areas in great detail. Others sweep across vast areas at low resolution.

Why the lines appear

Online maps stitch many of these surveys together, made by many different agencies and organizations, and stack them in layers. The result looks like one seamless picture of the ocean bottom — except that some places are mapped in much more detail than others.

Where a strange grid appears, you are seeing two or more maps on top of each other. The bottom layer is a broad, low-resolution survey, smooth and short on detail. On top of it lies a higher-resolution survey of a smaller patch, and its lines trace the paths of the ships that collected those sharper sonar readings. The streets of the "lost city" are simply where a survey ship went back and forth.

Sources

Based on "Did I find the lost underwater civilization of Atlantis?" from NOAA's National Ocean Service, a work of the United States government in the public domain.