The water in this area shallowed too quickly for it to get closer and it remained farther out in the dark water. Moments ago it had lifted its head above the surface, its cold dark eyes scanning the rocks for any movement – there was nothing – it had lost sight of them for now. However, above its cavernous maw, the lateral pores in its sensitive snout still detected their tiny footfalls on the rocks that sent the vibrations out into the water.

The Megalodon’s lifespan was long, much longer than that of its closest cousin the Great White Shark, and it could be patient. It could wait hours, days, or even weeks for the creatures to return. But moving into this area of the subterranean sea it had detected something else in the water. Far away it sensed miniscule movements – strange, constant, but getting louder as if something was approaching. It could be food, or it could be a threat.

With a few flicks of its huge tail it accelerated towards the new sensation. The more it travelled, the more the vibration increased in intensity. It was driven now by instinct – the strange sound might have been some creature in distress, and for the monstrous shark, that was impossible to ignore.

The Megalodon increased its speed, swimming hard now, and moving its massive torpedo-shaped bulk through the water at thirty-five knots, or just on forty miles per hour. Other creatures, some huge themselves, sped from its path, and were perhaps relieved it was not hunting them that day.

It caught up to the strange, bulbous creature that gave off a distinctive hum, and came up from the deep fast. It circled first, using all its senses to see, hear and taste the slow moving beast. There was little sign that the thing was edible, but its presence was a challenge that could not be ignored – it attacked.

The shell was too hard to crack, and then the thing defended itself inflicting pain and causing the Megalodon to spit it out. The shark circled as the thing sank slowly, motionless but not dead. Inside the shell it could detect the flutter of tiny heartbeats.

It prepared for another charge, when above it there came another maddening sound that seemed to come from within the stone roof itself. The Megalodon’s movements became ever more frenzied as something ground its way from the rock to poke down into the water. The massive eighty-ton, territorial beast accelerated to attack the new enemy.

It struck the end of the drill, further cracking the already weakened ceiling. Veins ran from the impact site in all directions, and then like some sort of colossal lid, it lifted away. It didn’t collapse and rain rocks down upon it, but instead the sheets of granite and silt were blown upwards by the subterranean pressure of the hidden sea.