Blue Planet II producer Orla Doherty reveals the moment her submarine sprang a leak

The best way to prepare for life in a submersible? Go to the loo… a lot.

It took an hour to get down to 1,000m and we would spend up to eight hours filming. That’s a long time without a loo break.

In total, I spent over 500 hours exploring the deep ocean.

We did a lot of dives with one particular ship that had two submersibles, which meant we had cameras on one and lights on the other, and that produced really beautiful, cinematographic results.

It gets very dark, very quickly down there.

And once you’re in that world, all you can see is what comes into your light beam. You’re in a place you’ve brought light to probably for the very first time. That’s one of the many things that flips your mind.

I spent ten years studying coral reefs, so I knew the top 40m of the ocean inside out…

…but in my entire time at sea it never once occurred to me to wonder what was in the deep. And it turns out that there could be more life down there than anywhere else on Earth. There are more species of coral in the deep than there are in the shallows.

We saw a lake on the ocean floor in the Gulf of Mexico. That was pretty head-spinning.

It’s a hyper-salty lake of brine, five times more salty than average salt water, which is why it settles in this crater 650m deep. The lake is like a witch’s cauldron, with smoky vapour flowing off the top of it. The whole scene is a gothic fairytale at its worst. It’s ultra-toxic and anything that goes in stands a high chance of dying immediately. The paradox is that it is surrounded by a dense bed of life – giant mussels are growing on the shores of this lake. I was desperate to go into the lake but we risked never being able to come back out of it!

We also saw an underwater volcano.

It’s not a volcano spewing hot lava and explosive rocks; what came out of a desert-like sea floor were massive bubbles of methane – each at least the size of a basketball – that rocketed up to the surface. It was like we’d landed on another planet.

There was a spot of trouble 450m down in the Antarctic…

The water was so cold one of the seals on a pressure gauge wasn’t absolutely sealed, so there was a small leak pooling at the bottom of the sub. It never ripped or ruptured, but it certainly had our hearts beating pretty fast for 20 minutes while we tried to find it and stop it. That was definitely my most “exciting” dive. We were very much at the point of no return, but we fixed the leak and we carried on.