Taking a dive in a research sub in the Gulf of Mexico

Scientists are diving to the bottom of the sea to research the potential environmental impact of the Deepwater Horizon oil spill in the Gulf of Mexico.

A mini-submarine stationed on the Research Vessel Seward Johnson is looking at the effect on the coral reefs 200 miles from the source of the spill.

One aim of the expedition is to measure the state of the reefs before any oil or dispersant may arrive - so that future changes can be measured. Another is to devise new techniques for assessing the vulnerability of coral.

David Shukman joined the team off the west coast of Florida as they prepared to go on a dive.