Media playback is unsupported on your device Media caption The aquatic life which has colonised the sunken container. Video courtesy MBARI

A shipping container lost off the coast of California in 2004 is now teeming with underwater life, say scientists who returned to it with a robotic sub.

However, it is unclear whether the "artificial reef" that the container provides is a beneficial one.

Containers could provide a "stepping stone" route for invasive species to colonise new areas, the team says.

The unique study sheds light on the problem of the estimated 10,000 such containers that are lost each year.

It is estimated that some 200 million shipping containers are used globally each year, and that at any one time, between five and six million containers are in transit.

Tyre tracks

In February 2004, the container ship Med Taipei set off from San Francisco.

What concerns me is that we might be changing this ecology before we even understand it Andrew DeVogelaere, Monterey Bay National Marine Sanctuary

Caught in a storm on its coastal journey to Los Angeles, 15 containers broke free and were lost near Monterey Bay.

Four months later, on a routine remotely-operated vehicle (ROV) dive by scientists from the Monterey Bay Aquarium Research Institute (Mbari), one of the containers was spotted at a depth of 1,300m.

The container - listed as housing more than 1,000 tyres - appeared to be in pristine condition, but the Mbari team resolved to return and assess the container's effects on life on the seafloor.

Last week, Andrew DeVogelaere of the Monterey Bay National Marine Sanctuary and colleagues from Mbari took the Doc Ricketts ROV back to the site.

"What's normally down there is gently rolling, soft seabed covered with an amazing amount of life - there are these clear sea cucumbers every few feet, beautiful pink crabs, tubeworms," Dr DeVogelaere told BBC News.

"Now into that you've dropped this hard substratum, this container, and on that you'll see other species like Neptunia, a large whelk that lays its eggs on the container, and it also seems some large crabs and octopus then move in and are feeding on the whelk."

Image caption The submersible Doc Ricketts returned to the site seven years on

While some efforts at "artificial reefs" elsewhere in the world's oceans change the biodiversity in a particular ecological niche, Dr DeVogelaere explained that it is unclear whether the container's presence presented a benefit.

"When you have large expanses of one kind of habitat, it can create a 'biogeographic break point' - you have species distributions that are abundant in one area of the coastline and then they disappear.

"What could be creating breaks - (preventing) invasive species moving from one area to another - is an expansive habitat, and in this case we may be creating stepping stones across that for the ecology to change. What concerns me is that we might be changing this ecology before we even understand it."

Given the growing preponderance of containers on the seafloor - some 10% of which may house materials toxic to marine life - the find has spurred the researchers to examine the shipping industry itself.

"They're going to be sitting in the bottom of the ocean for hundreds if not thousands of years, and building up through time," Dr DeVogelaere said.

"In one journal they describe containerisation like racecar driving: if everything's under control, you're not going fast enough.

"We'd like to understand the business, overlaying that with the biology and ecology. We want start to meld the thinking of the two so we're not doing science without understanding containers and so the container business is thinking about ecological impact."