My darling Ilse is pregnant. The news should fill me with joy, but the dates do not lie: it is Heider’s. Ilse is bereft; she calls it her abomination and wants me to end it, but I won’t do it. Her fever is so bad I fear she will lose the child anyway. I am terrified of this because a stillbirth out here will surely kill her. I curse the lack of equipment, the heat and the insects. These simple villagers have done their best to help us with food and shelter, but the few medical supplies we fled with are almost gone and I think I must return to the submarine. This has been the darkest of days.

* * *

Perhaps a world-class free climber might have found a way up the sheer and crumbling sides of the sinkhole, but for Nash, without a rope and pitons, it was suicide. And had there been any likelihood of finding usable rope inside the submarine, it might have justified a daring traverse of the log jam to reach the cracked hull. Which left the crescent pool as the only possible way out.

Nash warily eyed the still water. Free diving caves was known as sump diving, and it was one of the most dangerously stupid things you could do. He had retrieved several bodies of those who’d tried – most of them cocky teenagers on a dare who’d panicked and drowned in terror. But he also knew the submerged exit passage could just as easily rise up to fresh air and freedom.

Thanks to years of training for big wave hold-downs, Nash could hold his breath for a full three minutes under exertion. But there were other factors to consider. Diving alone in darkness, without a light, his autonomic nervous system would flood his system with adrenaline, gobbling up precious oxygen. The lack of a mask, weight belt and fins would compromise his efficiency and duration, and, as ever, there was the lurking threat of claustrophobia. He decided on an initial foray of two minutes, setting the timer on his dive watch for a one-minute turnaround.

Nash involuntarily shivered. In diminishing light, any kind of free dive was exponentially more dangerous. After three minutes breathing up, he slid into the chilly water. The smooth rock bottom steadily dropped away, and he swam along the arm of the crescent, getting his bearings. Descending to nine metres, he sensed the exit passage looming in the shadows. Then, the alarm went off, and he ascended with half a minute to spare.

So far, so good.

Nash stretched out like a lizard on the warm stone by the middle of the pool. Skin tingling, he began the breathe-up cycle again.