This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Editor’s Note: Veteran science journalist Philip Hilts is working with a team of archeologists, engineers and divers off the shore of Antikythera, a remote Greek island, where a treasure ship by the same name sank in 70 B.C. New, high-tech gear is allowing the team for the first time to examine and excavate a wreck with the care and thoroughness of an archeological dig. This is the second installment in a series, the entirety of which can be found by clicking here; or follow these links to thefirst and third stories (a table of contents can also be found at the end of this post).
Antikythera, Greece—The Antikythera wreck has been haunted from the beginning by the dangers of deep diving.
Georgios Kritikos, one of the sponge divers who brought up treasures in 1900 from the wreck 165 feet below the surface, was killed by decompression sickness during the work. Two more divers from the same team were paralyzed. During Jacques Cousteau’s trip to the wreck in 1953, one of his divers, Federic Dumas, for days had a paralyzing pain in one shoulder and arm from the same sickness.
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But the divers on this year’s expedition here have a technology that is relatively new to marine archeology, and in the right hands, offers a safer mode of diving.
The work alongside the remote Antikythera Island began September 15 and will continue to about October 12, depending partly on the notoriously bad wind and seas here, which is the same place that Homer wrote that Ulysses was pushed by wind and waves and lost his way home. The laden Roman ship that sank at this spot in 70 B.C. smashed on the submarine rock wall on the north side of the island, and dropped to a shelf on the wall. Attempts to dive to the wreck over the years have been hampered by the dangerous depth.
Each diver who steps off the stern of the dive boat this week has multiple gas tanks and a computerized breathing system called a closed-circuit rebreather. The gear allows the divers to get more than 20 times as much bottom time to work—more than an hour on each dive.
The first dive was to moor a buoy to a rock on the bottom so that the crew can find the exact spot day after day. Divers also began sweeping part of the site with an underwater metal detector. As they work, diver Alexandros Sotiriou says it’s necessary for them to keep part of their attention constantly on their depth and what their electronic gauges are saying.
Deep diving is dangerous because of the great pressure the water exerts on the human body. It compresses gases to a fraction of their usual volume, and squeezes tissues. The body compensates by using the air breathed in from tanks to fill in and resist the pressure. That means that the body must cope with a large volume of air, mostly nitrogen, packed into lungs and tissues.
The compressed nitrogen acts as an intoxicant, however, and the intoxication increases with depth and the time underwater. Divers use the “martini” metaphor to describe it: each 30 feet down produces the equivalent intoxication of one additional martini. At the first 30 feet the effect is not usually noticeable, but below that it increases precariously.
Limiting the time on the bottom is one obvious way to lessen the threat. The sponge divers in 1900, with their metal helmets and air hoses to the surface, allowed themselves only five minutes on the bottom before ascending again. The scuba rebreathers the diver on this expedition are using give them far more bottom time. In the first two dives this past week, the divers spent more than an hour on the bottom after each descent.
Instead of a simple metal bottle of air to breathe from, as in standard scuba, a rebreather draws from two cylinders, one containing oxygen and one a “trimix” cocktail of helium, oxygen and nitrogen. The rebreather takes the diver’s exhalations, scrubs out the carbon dioxide, then injects oxygen to replace what the diver has used. The software monitors the oxygen level and in effect makes a custom mix for all phases of the dive.
At 165 feet, the mix contains a high level of helium, perhaps 40 percent, to take the place of some of the nitrogen normally present in air. Displacing the nitrogen is important because that is the gas that can lead to narcosis, called by Jacques Cousteau “rapture of the deep.” It slows response time, reduces mental capacity and leads to poor decisionmaking. Helium is less narcotic.
Another danger is what happens when the dive is finished and the divers surface. Resurfacing with a load of gas compressed in body tissues is risky. Surfacing too fast quickly releases the pressure holding the gas in; the gas can emerge from tissues as bubbles in the blood that can block circulation in small blood vessels and cause severe damage to nerves and other tissues.
The resurfacing plan for Short and Sotiriou included stops to hover every few meters as they headed upward. Although they got down to the bottom in minutes, they took an hour and ten minutes to rise. Very boring, they say, but necessary.
The metal detector the divers are using is the second crucial piece of technology on hand. Because the statues and other objects from the ship have been in the salt water for more than 2,000 years, most of them have become “concreted” —encased in calcium carbonate and other minerals and buried in the sand and sediment. The detector can help find bronze and gold beneath sand and rock—as well as pinpoint where to look, in the first place.
From the water above, the wreck looks simply like white rocks and sand, with few obvious signs of a ship. The wooden hull has mostly dissolved, and what is left is buried and must be dug out. The detector’s coil creates a magnetic field that gets distorted when near a metal object. The distortion is measured and shows up as a signal on a meter and is heard as a whining sound through earphones. This week the divers have heard several strong signals of metal from beneath the sands, and hope to soon dig to the source.
It is clear that many valuables are still on the wreck, because previous salvage work has brought up dozens of parts of bronze and marble statues—heads, bodies, arms—and left the rest in place. In 1900, when divers were hastily digging out mostly small objects, one of the archeologists asked to pull up one of the big boulders just to check it. It turned out to be a totally concreted, a nearly 7-foot bronze statue of a young man believed to be either Paris or Hercules—which of course was undiscovered until workers chiseled off the outer casing. It is now one of the prize exhibits in the national museum in Athens.
The Greek nonprofit called Argo is the organizing body for the expedition, employing the divers, engineers and other staff. The chief goals for this inaugural year of the “Return to Antikythera” mission are to make a detailed and accurate 3-D photo map of the whole wreck site, and to then begin the painstaking archeological work of excavating the site, foot by foot. Dives are planned for the next several years, if funding permits, to complete the detailed archeological record.
Other posts in this series: