Series producer Helen Thomas on an ocean-going journey that was both difficult and dangerous.

BBC2's major new science series, Oceans, is an ambitious project: a diving expedition to eight seas and oceans - from the Arctic to the tropics - filmed in high definition over a year.

This was a natural-history/expedition/science documentary that had to be filmed on a tight budget and schedule - we had two weeks to shoot a one-hour programme in each ocean. Could it be done? No one was quite sure, given the vagaries of weather, boats and infuriatingly elusive sea creatures. There was a risk that we wouldn't come back with a programme.

We aimed to film about six to eight stories on each expedition - with the help of experts. This came in the shape of expedition leader and polar explorer Paul Rose, environmentalist Philippe Cousteau Jr, grandson of ocean pioneer Jacques Cousteau, Dr Lucy Blue, one of the world's leading maritime archaeologists, and marine biologist and oceanographer Tooni Mahto.

Over a year they voyaged across the globe, building up a picture of our changing seas and oceans. We wanted to pack the series full of science content alongside the expedition story, deciding to shoot in an actuality style to capture the sense of genuine exploration and discovery. That's relatively easy when filming above water, but underwater it's much more challenging. I don't know of any actuality team who would start the day aiming to film for just one hour. But that's often all you'll get out of a day's diving, especially if the dive is deep.

The production team was drawn from Factual London and Bristol's Natural History Unit. We also brought in dive contractor Richard Bull to help keep everyone safe underwater.

Our schedule didn't have a second to spare. We planned and planned, but were reliant upon a good deal of luck too. But we had a fantastic team, so if the conditions were right we knew that we would get the shots. Scott Tibbles led the camera team - he has that rare combination of skills: terrific topside actuality and great at underwater filming too.

The first shoot was in the Mediterranean. In June 2007, 18 people from the UK and the US, accompanied by almost two tonnes of luggage, set off to live on a boat for 14 days. That was the theory, at least. Our boat, Princess Duda, didn't turn up on day one; or days two, three, four and five. There was a huge rejig of the schedule, and the expedition began by loading and unloading the gear in and out of hotels, vans and onto small boats on a daily basis. A real test of a newly formed team.

In our first dive we were looking for a rarely seen prehistoric shark, the Six Gill Shark, in the Straits of Messina between Sicily and Italy. The Straits are a busy shipping lane with turbulent currents. And it had to be filmed at night - making it was dangerous.

The rescue boat had communications with the dive team and we were also recording sound underwater. All would be well as long as the comms kept working and we didn't lose the divers, which we did once for a few heart-stopping minutes. We had to turn all the lights out on the rescue boat and ask the divers to turn their lights towards the surface so we could find them again.

By night three we had almost given up hope of finding the shark, but in the last few minutes of the dive Paul found one and the opening sequence was captured in sound and vision.

Each expedition threw up different challenges. Sometimes the conditions were a hazard in themselves, like filming in the Arctic, which involved anchoring a boat onto the floating pack ice and diving beneath the ice.

Just getting permissions to film in Eritrean waters took 10 months, and while we were there we were accompanied by a member of their Navy at all times. From Eritrea we moved on to Sudanese waters and I was woken by a banging on the cabin door at 5.30am to be told we couldn't leave port until we had our rushes “checked” by the authorities.

While we were filming in the Indian Ocean our expedition boat was impounded, although we managed to negotiate our way out of that after a nail-biting 24 hours. We were boarded frequently, relentlessly by various authorities, some toting AK47s.

The whole team was brilliant - they've overcome bizarre challenges, been in very hairy situations and worked incredibly hard to make a series that I hope will reveal some of the ocean's more extraordinary stories.
Oceans is an 8 x 60-minute BBC/Discovery HD co-production. It airs on BBC2 on 12 November at 8pm

Helen Thomas: My tricks of the trade
Sea sickness pills: top of everyone's list on a dive boat, along with wristbands, ginger... we tried everything.

Waterproof paper: everything gets wet and even if it's only the treatment that's printed on waterproof paper it's worth having.

Head torch: what a brilliant invention - they make you look ridiculous, but they're absolutely invaluable when you're clambering around a boat in the dark in rough seas.

Flapjacks: stuck on a small boat for hours, waiting for the deepest diving mammal on the planet to resurface - flapjacks kept everyone motivated.

Uno: when you're living in each other's pockets 24/7 there needs to be a really good way of releasing any pent-up angst and this card game is it.