Leviathan

He padded to the back of the cave, peering down into the deep watery hole before him. To call this the end of the cavern system would be inaccurate; the caves went for miles further beneath the surface of this inky pool. He was determined to explore them and haul up the treasures he found to build his hoard.
He had been practicing deep drives for weeks now, determined to survive the watery tunnel. He wasn't sure when or even if there was an air pocket, so he had to be prepared. He'd gotten pretty good by now, able to hold his breath for hours.
He had told no one of his plans. This was in part because he didn't want anyone to try and stop him. However, his greed for the treasures of this subterranean trove were also a key factor; he did not want to share them.
He took a couple deep breaths to prepare himself. His lungs filled and stretched only to be emptied as completely as possible. Then, he dove in.
The dark, cold water enveloped him. It used its grasp to drag him deep into itself. He couldn't see or breathe, but that was to be expected. He clawed his way downward in the dark. The stone scraping at his sides was his only indication of the confines he was in.
As his claws scraped rock infront of- or was it below?- him, he felt one wall disappear. With a tight grin he pulled himself through the bend. This was only the beginning of his journey and he looked forward to every stroke. He only hoped that his lungs held enough air to see him through to the other side.