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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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