When you are offshore and looking at empty water stretching all around, the odds of running into something barely visible seem very slim. Yet the hazard is common enough to warrant its own yachting acronym in France. There they are called UFOs: unidentified floating objects, and chief among them these days are shipping containers lost at sea.

Sometimes these UFOs are marine creatures, basking sharks or whales mostly, identified perhaps by blood in the water from the injured animals. It’s sad and gruesome, dangerous on both sides and very difficult to prevent. (Read more about the risks of collisions with whales.)

Statistically, though, perhaps a greater risk is from semi-submerged container lost overboard from a ship. In March, the Office of National Marine Sanctuaries, a department of US oceanography and weather body NOAA, published an extensive report on the loss of shipping containers worldwide. It makes for chilling reading.

The report notes that containerised maritime trade ‘grew eight-fold from 1985 to 2007, and worldwide there are now approximately 5 to 6 million containers in transit at any given moment.’ An estimated 10,000 shipping containers fall from ships every year.

The reasons for these losses are rough weather, inadequate or faulty securing systems, and (quite often) miscalculations of container weights when a ship is being loaded. The latter was a contributory factor in the loss of the MSC Napoli off the Devon coast in 2007. Around a fifth of the 660 containers on the deck of the ship were found to be heavier than their declared weights, some by as much as 20 metric tonnes.

I had always assumed that anything but an empty container would sink reasonably quickly if it were lost overboard, but that’s not the case, and the report has some worrying conclusions. ‘Following loss incidents, containers rarely sink immediately.

Depending on whether they are full or empty, and on the nature of the cargo inside, containers may float at the surface for several days or weeks prior to sinking. Containers are not generally entirely watertight; while an empty container is likely to sink due to water ingress, a full container will likely float until air trapped in the cargo has escaped.’

A New Zealand insurance company has calculated that a 20ft container would have to exceed 16 tons before it sank, and a 40ft container would have to exceed 32 tons.

So for quite a long time lost containers may float with most of their surface below the water, exactly like a growler. But these are growlers that lurk unseen along the world’s busiest shipping routes.

The problem is growing because ship sizes are increasing. As each generation of ship becomes larger, the stacks of containers grow taller. At the moment, the average container ship is 5,000 TEU (TEU means ‘twenty-foot equivalent units’). But when the Panama Canal expansion project is finished next year, the canal will be able to handle ‘Post-Panamax’ container vessels that can carry 12,000 TEU. And these will not be as large as the latest generation of ‘Malaccamax’ ships designed for transiting the 25m deep Strait of Malacca, which have a capacity of 18,000 TEU. If only a tiny fraction of these containers are lost, the totals are enormous.

Aside from the navigational hazards and the economic costs, the impact on the environment is significant. This report says that 10,000 lost containers equates to 41,500 tons of littered steel in container weight alone and ‘it is conceivable that 100,000 tons of substances in packaged form – many of which may be harmful – are falling off ships in containers each year.’

So what is the answer? Maritime and shipping organisations are looking at better stacking systems and procedures, more efficient ways of verifying container weights and methods of identifying lost containers. A proposal to the IMO a few years ago was to equip containers with a beacon. With technology that is miniaturising and getting cheaper all the time it’s possible that in future these man-made UFOs could show up on AIS.

The whales, though, will be trickier to tag.