Sarah and Tim loved swimming. They were both in the school team, and went to the same diving club, down by the docks. Although only twelve, they knew how to handle themselves both in and under the water.

Sarah signalled to Tim that they had ten minutes air left in the tanks, and he looked up towards the bottom of the boat to check that they hadn’t drifted too far. He saw the familiar bright blue hull of the ‘Pride of Dawlish’ above them, and gave her the thumbs up.

The pair swam around the edge of the next rock pile, enjoying the mesmerising movements of the small fish and pointing to the different crabs they saw on the sandy floor below them. Once around the pile, Tim glimpsed an entrance ahead, and pulled Sarah by the arm to go and investigate. Once inside, they realised that it was cavernous, and soon needed their torches to see ahead. It was then that Sarah noticed their air was unexpectedly low, and they turned gracefully to return to the surface. They looked ahead – where was their tunnel? There seemed to be many where there had been only one! How would they know which one to take? Their air was too low to investigate them all.

Seeing a warm light above, Tim swam upwards, until the pair appeared inside an enormous underground chamber – big as a stadium. The pair looked about them, open mouthed. They could not believe what they had found.