1531

DIVING BELL DEBUTS
Treasure hunters don crude diving bells to scour the bottom of a lake near Rome for two bejeweled ships built for Roman emperor Caligula. The barrel-shaped bell is worn over the neck and shoulders, trapping enough air for divers to search for up to an hour.

1620

FIRST SUBMARINE BUILT
A dozen men power the world's first submarine with oars protruding through holes in the sides of the vessel that are sealed with leather. Dutch inventor Cornelis Drebbel conducts trials in the Thames River for the king of England.

1691

BARRELS DELIVER OXYGEN
Englishman Edmond Halley, better known for Halley's Comet, extends the air supply for diving bells by sending down weighted barrels of oxygen as refills.

1715

SUIT SUFFERS FLAWS
Englishman John Lethbridge creates an early diving suit that looks like a horizontal barrel with armholes sealed with leather. He fails to reach great depths because of a difference in pressure between his torso and limbs.

1788

DIVING BELL GETS UPGRADE
American John Smeaton adds an air storage tank fitted with a hand pump that pulls air through a hose to the surface. A one-way valve prevents air from returning up the hose.

1864

STEEL LUNG INSPIRES SCUBA
Frenchmen Benoit Rouquayrol and Auguste Denayrouse patent the Aerophore, an early form of scuba. A diver inhales air from a tank through a membrane and valve. Air at the surface is pumped down a hose into the tank to refill it.

1870s

REBREATHER RECYCLES AIR
Englishman Henry Fleuss invents a rebreather, a closed-circuit system that supplies compressed oxygen to the diver while absorbing exhaled carbon dioxide, using a rope soaked in caustic potash.

1900

NAVY BUILDS SUBMARINE
Irish immigrant John Holland constructs a modern submarine powered by gasoline and electricity for the U.S. Navy. His model, the Holland, quickly catches on, and similar vessels debut in navies worldwide during World War I.

1919

WAR SPURS SONAR
The British and French navies devise a sonar system to detect submarines. It is installed on destroyers over the next two decades.

1934

EXPLORERS SINK IN STEEL BALL
Two men inside a two-and-a-half-ton hollow steel ball called a bathysphere set a diving record of a half-mile deep. A steel cable from a mother ship lowers and lifts them, a second cable powers a telephone and lights, and a tube provides air.

1943

DIVERS BREATHE DEEPLY
The Aqua-Lung goes on sale, fashioned by explorer Jacques Cousteau and engineer Emile Gagnan. This scuba technology incorporates an automatic demand valve to supply fresh air to the diver with each breath. It remains the basic system that scuba divers use today.

1955

SEARCH FOR METALS FINDS PLATES
Scientists survey the seafloor using an underwater metal detector called a magnetometer, towed behind a ship. The surveys reveal a striped pattern of subsurface metals that date the sea bottom and provide evidence for the theory of plate tectonics.

1960

HUMANS REACH MAXIMUM DEPTH
The U.S. Navy's Trieste, a 6.5-foot-diameter bathyscaphe with a 50-foot tank of gasoline on top, reaches the deepest point in any ocean, the bottom of the Mariana Trench seven miles down—despite a cracked window.

1960s

SCIENTISTS DEPLOY ROBOTS
The U.S. Navy funds development of remotely operated vehicles: underwater robots that researchers on a ship above maneuver by remote control to collect data or virtually explore shipwrecks.

1970s

MACHINES GO IT ALONE
The University of Washington deploys an autonomous underwater vehicle to the Arctic. It is preprogrammed to gather data and complete tasks without a human operator. Progress with autonomous and remotely operated vehicles has brought 98 percent of the ocean floor within reach of scientists.

1980

VIDEO EMERGES FROM BELOW
Robert Ballard, discoverer of the RMS Titanic, creates an underwater camera that streams live video via optical fiber to the surface for scientists and educators to watch.

2014

DIVERS FIND NEW FLEXIBILITY
A team excavating a shipwreck in Greece tests the Exosuit, an armored suit that maintains sea-level pressure inside, allowing divers to reach depths of 1,000 feet for up to 40 hours.

SCIENTIFIC AMERICAN ONLINE For a blog and photographs from the Antikythera expedition, go to ScientificAmerican.com/jan2015/hilts