The First Explorers: Breath-Hold Diving
Long before any technology existed, humans were already exploring beneath the ocean's surface. Evidence of breath-hold diving (also called free diving or apnea diving) stretches back thousands of years across cultures worldwide. In ancient Greece, divers harvested sponges, pearls, and shellfish, and were even employed in military operations to sabotage enemy ships or recover anchors.
In Japan and Korea, the tradition of the Ama divers — predominantly women — has persisted for over two millennia. These divers descend without equipment to depths of up to 30 metres, holding their breath for up to two minutes, harvesting sea cucumbers, abalone, and urchins. The practice continues today, though in smaller numbers.
The Diving Bell Era (16th–19th Century)
The first mechanical aid to underwater exploration was the diving bell — an open-bottomed chamber lowered into the water, trapping air inside. Early versions were simple wooden or metal chambers with no air replenishment, limiting a diver's time at depth. In 1691, Edmund Halley (of Halley's Comet fame) designed an improved diving bell with a system for replenishing air via weighted barrels lowered from above, allowing divers to remain underwater for longer periods.
By the 18th century, diving bells were being used for salvage operations, harbour construction, and the inspection of ship hulls — a significant practical advance, though still severely limited by depth and duration.
Helmeted Diving Suits (19th Century)
The invention of the standard diving dress — a waterproof suit connected to a surface air pump via a hose, with a heavy metal helmet — transformed underwater work. Augustus Siebe's design, refined in the 1830s, became the standard for commercial and naval diving for over a century. These "hard hat" divers could work at depths previously unreachable, enabling major engineering projects: harbour construction, cable laying, and salvage of wrecked vessels.
The limitation was the tether to the surface — divers could not move freely or explore at any real distance from their entry point.
The Age of the Bathysphere and Bathyscaphe
The 20th century brought the ambition to reach the truly deep ocean. In the 1930s, naturalist William Beebe and engineer Otis Barton developed the bathysphere — a spherical steel chamber lowered on a cable from a ship. In 1934, Beebe descended to 923 metres off Bermuda, the deepest a human had gone at the time, and broadcast live descriptions via telephone to audiences above.
The next leap came with the bathyscaphe — a free-floating deep-sea vessel designed by Swiss physicist Auguste Piccard. His design, using a buoyant float filled with gasoline (less dense than water) and iron shot for ballast, allowed independent movement at depth. In January 1960, his son Jacques Piccard and US Navy Lieutenant Don Walsh descended to the bottom of the Mariana Trench in the Trieste — reaching approximately 10,916 metres and demonstrating that life existed even at the ocean's greatest depths.
SCUBA: Exploration for Everyone
The development of SCUBA (Self-Contained Underwater Breathing Apparatus) by Jacques-Yves Cousteau and Émile Gagnan in 1943 democratised shallow-water exploration. The aqualung, as Cousteau called it, freed divers from surface connections, allowing them to move freely through reef and wreck environments. Cousteau's subsequent films, books, and television series brought the underwater world to a global audience for the first time.
ROVs and Modern Deep-Sea Science
Remotely Operated Vehicles (ROVs) emerged in the 1960s and have become the primary tool of modern deep-sea science. These tethered, camera-equipped robots can be operated from surface ships for extended periods, reaching any depth in the ocean. They have explored hydrothermal vents, surveyed shipwrecks, and collected biological samples from environments hostile to any human presence.
Autonomous Underwater Vehicles (AUVs) go even further, operating without a tether and following pre-programmed routes to map the seafloor, measure water chemistry, and document ecosystems. The combination of ROVs, AUVs, improved sonar mapping, and genetic analysis of seawater samples (environmental DNA, or eDNA) is revealing the deep ocean faster than ever before.
What Still Remains Unknown
Despite all of this progress, scientists estimate that the vast majority of the ocean floor remains poorly mapped and biologically uncharacterised. The deep ocean is the largest habitat on Earth, and it continues to yield surprising discoveries — new species, unexpected geological features, and ecosystems we did not imagine could exist. The history of deep-sea exploration is far from over.