Maritime authorities were still cleaning up two days after 22 shipping containers fell off the 91-foot barge Atlantic Trader Monday afternoon about 18 miles east of Key Biscayne, Fla.

The tug Spence told the Coast Guard that 22 of 129 containers toppled into the sea while the vessel was en route from Jacksonville, Fla., to Guantanamo Bay, Cuba.

An air crew from Coast Guard Air Station Miami found some of the containers floating near the Spence. Coast Guard cutter Gannet crewmembers marked the containers with strobes as hazards to navigation.

By Wednesday, only one of the containers was still visible; the remaining 21 were presumed to have sunk.

Click play for another close call by a container ship.

“Our air crews searched extensively following a drift pattern, just like with a missing person,” Petty Officer Jon-Paul Rios of the Coast Guard District 7 Public Affairs office told Soundings. “It’s safe to say the rest have sunk, and the remaining container is expected to sink before commercial salvors reach it.”

As of Wednesday afternoon, the strobe-marked container was 27 miles off Fort Pierce, and mariners were urged to remain vigilant in the area.

Of the 22 containers, only five contained anything close to being deemed hazardous material, Rios said — only marginal threats to the environment, such as paint.

The cause is under investigation.

Striking a shipping container lost at sea — in addition to weather and collisions with whales — is one of the few hazards that boats that venture offshore face. There have been numerous accounts of cruising and racing sailors striking containers offshore, sometimes with disastrous outcomes.

Estimates vary from as many as 10,000 shipping containers lost each year to as few as 2,000.

The World Shipping Council has taken issue with both of those figures and, in a study, said there were “approximately 350 containers lost at sea each year, not counting catastrophic events. When one counts the catastrophic losses, an average total loss per year of approximately 675 containers was observed.”

Most containers are not watertight and sink, but some can float for years — sometimes just below the surface — presenting a danger to cruisers and racers, as well as commercial vessels.

Click here for a spirited discussion about the threats to bluewater cruisers at CruisersForum.com.