By Dr David Whitehouse
BBC News Online science editor
The latest trip to the Titanic suggests that tourists visiting the site are causing much more damage to the wreck than its deep sea environment.
"Rusticles" drip from the railings
That is the conclusion of a new survey of the 3,600m-deep (12,000ft) wreck by robots.
Mission scientist Dr Robert Ballard, who discovered the Titanic in 1985, says the visits are taking their toll.
"Submarines come for salvage work, filming and tourism, and they have been landing on the deck of the Titanic where they are causing damage," he said.
The 2004 Return To Titanic expedition, co-sponsored by the US National Oceanic and Atmospheric Administration (Noaa), has been a difficult one, plagued by bad weather and technical problems with the unmanned, remotely operated vehicles.
When conditions improved, engineers from the Institute for Exploration, headed by Dr Ballard, sent underwater robots on a four-hour dive to the great ship.
The expedition spent 27 May to 12 June positioned over the wreck.
The port side was surveyed
The Titanic sank on 14 April 1912, after hitting an iceberg on its maiden voyage. More than 1,500 people lost their lives, while more than 700 passengers and crew were rescued.
"It was important to go back to Titanic," says Captain Craig McLean, director of the Noaa Office of Exploration.
"It's a cultural icon and a maritime gravesite deserving of our respect. But it's also a deep-sea laboratory where we can study the chemical, biological and human effects on the ship's rate of deterioration and apply that knowledge to other deepwater shipwrecks."
The latest mission conducted a high-definition survey of sections of the ship's hull, in particular the bow and the stern.
The expedition's chief archaeologist, Jeremy Weirich, said: "Additional research was conducted on 'rusticles', so-named because they appear to be rusty icicles but in fact are communities of microbes that process Titanic's iron."
Scientists also recovered two platforms containing a variety of metal test strips that were placed in the vicinity in 1998.
A main objective was to obtain a photo-mosaic of the ship, to compare it with photographs Dr Ballard took nearly 20 years earlier.
"We are finding very little has happened from natural causes," said Dr Ballard. "The ship is very similar to the ship we investigated 18 years ago, except where the submarines have been landing."
A human tragedy
The protection of the Titanic took a major step forward recently when the US ambassador in London signed an international agreement that will lead to an enforcement authority to protect the site.
The new authority will replace voluntary guidelines that have been in place since 1986 which scientists believe are ineffective.
Under the new directive, the Titanic will be designated an international maritime memorial to those who perished there. It will also regulate visits within the jurisdiction of the signatories - the US, Canada, France and the UK.