Incredibly, there was a muffled return cry.

‘Rob, are you there?’

Its source was a small, ragged hole in the bulkhead near floor level. Boerman had also severed a gate valve, designed to give crew trapped underwater a chance of escape under pressure by flooding the compartment. It had been done so Mia would slowly drown.

‘Mia,’ he called though the hole.

‘Rob, where are you?’

He directed his dive light carefully through the hole.

‘I see you!’ she shouted. ‘Oh God, help me!’

‘Mia, listen – can you open the door?’

‘I’m tied to something! I’m trying to get free, but these cable ties are cutting me to pieces.’

Nash pressed his forehead to the cold steel. Even if he found a way to plug the hole and slow the water down, the door would soon be impossible to open. Mia’s only hope was to escape via the top bulkhead door, but in order to reach it, she needed to be able to climb.

‘Rob? Are you still there?’

Somehow he had to reach the top bulkhead door from the other side and climb down to free her. The only access was via the hole Boerman had cut in the hull, which was twenty metres above their heads, but the Afrikaner knew full well it was his only option, and would simply pick him off.

‘Rob, I’m scared. Don’t leave me!’

‘Mia, listen to me. I’m coming to get you from the other side, but you have to find a way to get free.’

‘I’m fucking tied up,’ she sobbed angrily. ‘Didn’t you hear me?’

‘When the water comes in, stay with the air, all the way to the top. Breathe to the very last lungful, do you hear me? And then hold your breath for as long as you can. Because I might have to let the pressure equalise before I can open the door.’

Nash’s ears popped as air was squeezed into the end of the compartment by the rising water. It was close now, gurgling and hissing, finding its way into every crack and crevice.

‘Rob, I can’t get free!’ she shrieked. ‘Just shut up and say goodbye!’

The insistent water was sloshing around Nash’s neck. He was choking with grief and impotent rage, but he knew that if she was to have any chance, he had to stay calm, right to the very end.

‘Mia, you’re going to get free and I will see you at the top. One more thing . . . Do Not try and open this door or the water will crush you – do you understand? You have to go up. You can do it. I know you can.’

‘Rob –’