The current seemed slow, but in fact it didn't take long for the boat to drift to a point where the hippo was out of sight. Squinting at the other side, Ben thought he saw a suitable place to land, so he carefully knelt up once more and started paddling again, while Halima attempted to bail out some of the excess water.

They continued like this for a couple of minutes.

It was just as Halima had her hands over the side that she suddenly screamed. Quick as lightning, a crocodile had risen to the surface and snapped hungrily at Halima's arms. It was twice the size of the younger croc they had seen earlier, its skin horny and green-grey, and it was ten times more aggressive. Halima pulled back just in time, and the crocodile made another attempt, this time splintering the side of the boat with its razor-sharp teeth.

'Get back!' Ben shouted. This time he knew he had to do something; the crocodile was attacking, and clearly wouldn't just slip away like the hippo had. Adrenaline burning through his veins, he whacked it on the head with the flat part of his oar.

It did nothing. The croc attacked once more, splintering another part of the side of the boat.

Halima screamed again. Ben felt like screaming too, but what would that do? He hit the crocodile for a second time, gagging as the smell of rotting flesh from its teeth reached his nose.

It just seemed to make the vicious reptile more angry.

The third time Ben hit it, he managed to get the eye, but the oar was too flat for him to be able to poke it properly. He started to panic. One more bite of the side of the boat, he reckoned, and they would start to sink.

Ben's head started to spin, and he found himself hyperventilating. His instinct was to grab Halima, to protect her from the crocodile's next attack, though what he really thought he could do he didn't know. The beast had submerged itself somewhat, and there was a terrible silence as the pair watched its massive body curl round and swim away. For an instant Ben felt shuddering relief, but that soon disappeared as he realized what the croc was really doing.

It was preparing for another attack.

Ben knew what crocodiles did – it was the stuff of playground horror stories. Once it had one of them in its jaws, it would disappear with them to the bottom of the river until the struggling had stopped. Then it would store them in an underwater hiding place until the meat was slightly rotten before eating it.

It seemed to happen in silent slow motion. The water above the animal parted as it headed towards them, picking up speed as it came and opening its mouth, gaping wide.

One snap of those jaws and they would be crocodile food.

It was two metres away. Ben raised his oar.

Just as the crocodile was upon them, there was another movement in the water. For the second time in only a few minutes, Ben saw the boulder-like hump of a hippo emerge to the side of the crocodile. With surprising speed for such an ungainly animal, the hippo opened its lumpy jaws. The hippo's teeth were not as numerous as the crocodile's, nor as sharp; but they were huge and strong, like elephant tusks. As they closed round the body of the crocodile, the reptile started flailing in sudden pain and panic. Its lizard-like tail swung up in the air, showering Ben and Halima with a torrent of water. When they had wiped their eyes, they were only just in time to see the hippo submerging itself, its almost dead prey still clamped firmly in its prehistoric jaws.

There was no time to be relieved. The bulk of the hippo and the frenzied wriggling of the croc had caused the still water to become treacherous; and the holes the reptile had ripped into the side of the boat were allowing water to gush inside. Ben plunged the oar back into the river, and with all his might started sculling towards the bank; but they were sinking fast. It was clear they weren't going to make it.

'We're going to have to swim!' Ben shouted.

'I cannot!' Halima cried.

Ben sized her up. She was about his height, but slighter of frame. It was perhaps fifteen metres to the shore; he was going to have to carry her. It was that or let her drown.

Quickly he pulled his saturated shoes from his feet, and with all his strength threw them to the shore. 'Give me yours,' he told Halima; when she did so he threw them ashore too.

They had less than a minute before the boat was completely submerged. A sudden calm fell over Ben; he knew exactly what he had to do. He crouched down. 'Climb on my back and hold onto my shoulders,' he instructed Halima, whose eyes were now wide with fear like he had never seen. She did as she was told. 'Not so tight round the neck,' Ben said. Halima loosened her grip, but only slightly. Neither of them said what they were both thinking: that there could be anything between them and the shore. They were just going to have to trust to chance.

Ben had intended to hurl himself from the boat – that would have given him an extra couple of metres' start; but suddenly he became aware that the bottom of the hull had disappeared from beneath his feet. Halima's weight sent him under, and he kicked as hard as he could to bring them back up into the air, where they both spluttered. Then he started to swim.

The current was stronger than he had anticipated now that he was in the water, so he had to head on a diagonal towards the bank. The strain of going against the flow, together with the weight of Halima behind him, meant that soon the muscles in his arms, along his back and into his legs were burning with exhaustion. Every few strokes he would find himself going under, and he had to gather all his remaining strength to push the two of them back up to the surface of the water.

Ten metres to go.

Five metres.

The pain in his arms was too much. He was sinking. He did his best to gather his energy for one final surge up above the water, but it simply wasn't there. He was going down. He closed his eyes and held his breath.

And then his foot hit the bottom of the river. It felt hard, stony and slippery beneath him. Halima was wriggling and struggling on his back, but she kept holding on tight – too tight, around his neck. Ben looked up and opened his eyes to see sunlight streaming through the surface of the water. It was close. They couldn't be far from the edge now. Battling against the current, he took a step through the water. It was like wading through treacle, but somehow he managed it. And then another step. And then, if he stood on tiptoes, he could just get his head above the water. He gasped loudly, filling his air-starved lungs with a deathly rasp.

Halima was still on his back, so her head was already above water. 'Put me down,' she ordered. 'I can walk from here.'

Ben did as she said. And then he made the mistake of looking behind him. His eyes just above the level of the water, he could see the telltale mounds of crocodiles in the middle of the river, like floating logs. 'Hurry up,' he gasped. 'We have to get out of the water.' They waded towards the shore, urgently trying to get there as quickly as possible, but frustrated by the resistance of the water. It seemed to take for ever.

Gradually, though, the water became suddenly less deep, and they were able to run out, desperate to put distance between themselves and the circling crocs, despite the fact that the stones were cutting into their bare feet. Quickly Ben gathered their shoes; but once he had done so, he felt his legs collapse, jelly-like, beneath him. He was dizzy with exhaustion.

But Halima would not let him sit down. She started pulling at his arm: 'We have to get away from the river, Ben. The crocodiles are coming!' Ben looked out over the water to see she was right; the mounds were not so far away now, and getting closer. He forced himself to stand up, and then the two of them ran, still holding their wet shoes, behind the trees that lined the river.