River of Ghosts — Peter Tonkin

The exciting new Richard Mariner marine adventure - Richard and Robin Mariner are testing the unmanned sub Neptune when a distress call comes in from a submarine whose mission is to recover the lost gold of Kublai Khan. It has become trapped on the seabed and the crew is running out of air. But as the Mariners guide Neptuneto the rescue, it becomes plain that the voyage was a cover for a more sinister mission . . .