It is fiendishly difficult to locate these wrecks. The Titanic, for example, proved tantalisingly evasive, even though the searchers knew its location when it ploughed into the iceberg. None of these masses of steel - some, like the Titanic, 4,000 metres down - betray their presence with pollutants on the surface of the waters above.
So why not consider our deepest oceans as burial grounds for all manner of materials, if they are not going to pose any measurable pollution risk? This was the topic explored yesterday by Dr Martin Angel, a senior biologist at the Institute of Oceanographic Studies. Dr Angel was speaking at a one-day conference on the theme "Can ecological innovation sustain us?" organised as part of the British Association for the Advancement of Science's annual meeting, held this week at the University of Newcastle.
Dumping materials in the deep oceans may not seem a very sophisticated approach. "But," says Dr Angel, "with the human population set to double by the middle of the next century, even with recycling and waste minimisation, our waste-disposal problems will increase. Deep oceans - but not our shallow seas where 96 per cent of our marine living resources are - could be used to dump large concrete and metal structures, sewage sludge and even low-level radioactive waste, providing the site was capped with inert soil." More dumping on land is not an attractive alternative, given the danger of groundwater becoming polluted.
Oceans deeper than 2,000 metres cover 65 per cent of the world's surface. Most contaminants reaching those depths stay put and do not rise to surface waters. "No impact on biodiversity is likely to occur because so very few of the oceans' species live in these depths," says Dr Angel. "We know enough about our deep oceans to carry out a large experimental dumping, which must be monitored."
Not everyone agrees, as became abundantly clear when Greenpeace forced Shell to abandon plans to sink the Brent Spar oil-storage structure in the Atlantic. The British government had previously approved the oil company's plan to dump the 14,000-tonne structure in 2,000 metres of water between the Hebrides and Rockall at the North Fenni Ridge.
Brent Spar currently lies in Erfiord near Stavanger in Norway awaiting an independent inventory of its contents and a decision on how it is to be disposed of.
Though much of the Brent Spar debate was highly emotive, Greenpeace based its campaign on a number of facts. Shell had no detailed inventory of what substances the structure contained. It was impossible to be sure of their impact on the deep-sea environment (one of the most difficult ecosystems in the world to study). And deep-sea dumping could create an all-too-important precedent. Recycling, treatment or containment of wastes is a more environmentally acceptable alternative, and disposal after dismantling on land is technically feasible. Engineering firms are ready to take on the task.
Objectors feared the ocean-dumping of Brent Spar would not only open the gates to widespread dumping of old oil and gas rigs, but make general ocean-dumping of wastes - from industries other than offshore oil and gas - more acceptable. Surprisingly, the first possibility is less serious: the oil-and-gas industry is not seeking to dump each and every structure. Only a minority of oil or gas rigs would even be considered for deep-sea disposal. And Paul Horsman, Greenpeace's head of oil campaign, acknowledges that "it is unrealistic to remove some items such as sub-sea concrete structures on the seabed".
What most troubled objectors was the precedent that would be set for other wastes. Dr Helen Wallace of Greenpeace's science unit believes that sinking Brent Spar could have made the deep-sea dumping of other low-level radioactive and hazardous wastes seem more acceptable. These are not subject to minimum safe-disposal levels established by international agreements such as the London Convention.
Greenpeace is not alone in questioning the basis for Shell's proposals. In a letter to Dr Wallace written in July, Drs John Gage and John Gordon of the Scottish Association for Marine Science (Sams) expressed broad agreement with Greenpeace's practical concerns. They also disclosed that Sams scientists - who have considerable knowledge of deep-sea ecosystems around the proposed dump site - were never consulted for information. In addition, they questioned the claim that the deep-sea environment is poor in species, especially where invertebrates in deep-water sediments are concerned.
Brent Spar consists of 6,700 tonnes of steel, 6,800 tonnes of iron-ore ballast and 1,000 tonnes of equipment (ranging from pumps to batteries). It also holds other metals, including much zinc, copper and cadmium, plus wood, plastics and rubber, oily sludges, low-level radioactive material, silt and seawater.
Writing in the journal Nature in June, Professor Euan Nisbet and Dr Mary Fowler of London University argued that the quantities of heavy metals in Brent Spar are minuscule compared with those found naturally in parts of our deep oceans. In so-called "black smokers", whole communities of highly specialised bacteria eke out their lives where the earth's crust vents huge amounts of superheated water and metals into the ocean depths. One estimate puts this discharge of metals worldwide at 700,000 to five million tonnes a year.
Dump the Brent Spar near such a vent, argues Professor Nisbet, and its impact could not even be measured. Its contents might even give the bacteria a free meal. Shell's proposal, though, did not involve such vent areas. The North Fenni Ridge does not contain them, though Professor Nisbet claims its seabed is probably iron-rich. Disposal on land, he says, would be difficult because heavy metals such as cadmium are toxic to most land species.
These views are not universally accepted. A number of other scientists, from universities at Cambridge, Southampton and London, believe Professor Nisbet is exercising too little caution. A cocktail of chemicals, they argue, might upset the ecological balance in these specialised bacterial communities. You cannot simply argue that the quantities are small - much might depend on the mix and the precise chemical form they are in.
According to the UK Offshore Operators Association, more than 200 oil and gas structures have been installed in the UK sector of the North Sea since 1967 - half of all the installations there. Their weight ranges from 1,000 to 200,000 tonnes or more.
The industry has so far decommissioned nine structures, removing them entirely. Over the next decade, some 50 UK installations will go; nobody knows how many might be proposed for dumping. International Maritime Organisation standards require the entire removal of installations in shallow waters.
What Martin Angel was proposing at yesterday's conference was a large- scale trial to monitor whether deep-sea disposal is a safe option for a range of materials.Not everyone, though, might accept a "yes" verdict. What many environmentalists find irksome is the "out of sight, out of mind" mentality of deep-sea dumping by oil companies that have made substantial profits out of the North Sea.
But what is more worrying is that the debate over dumping may distract from the real problems faced by the world's seas and oceans. Overfishing, and instituting workable measures to prevent it, is of far greater concern. "On a Richter scale of ocean damage," says Professor Nisbet, "fishery problems would rate 10 and the Brent Spar 0.1."