Chapter 1 · Try it yourself

Sonar Mapper

In 1950 nobody knew what the floor of the Atlantic looked like. The trick was sound: send a ping down, time the echo, and do a little arithmetic. You are the survey ship. Sail from North America to Africa and ping as you go. What is down there?

Side view of the North Atlantic. Seafloor unknown. Start pinging. N. AMERICA AFRICA Depth 1,000 m2,000 m3,000 m4,000 m5,000 m6,000 m

Sounding (one ping)Seafloor you have mappedVertical scale stretched about 170 timesOn a small screen, swipe the picture sideways.

About 100 km between stops; the whole trip is about 6,000 km.

Ping sends a burst of sound straight down and times how long the echo takes to come back. Auto-survey sails east and pings at every stop.

Real history: in the early 1950s Marie Tharp was turning rows of echo soundings like these into seafloor profiles at Columbia University. In 1952 she noticed a deep notch running right down the middle of the ridge in every profile: a rift valley, where the seafloor is being pulled apart. Her colleague Bruce Heezen at first dismissed it as “girl talk.” She was right.

At the coast of North America
Depth: not measured

Echo time: press Ping

Pings so far 0 of 61

Features found

    Simplified model: a made-up but realistic North Atlantic profile, with soundings about 100 km apart and sound traveling at a steady 1,500 m/s. Real sound speed varies a little with temperature, salt and pressure. The real rift valley is only a few tens of kilometers wide, so it is drawn wider here, and all heights are stretched.