What is one example of imagery from the story, Through the Tunnel?

From the text:

A hundred, a hundred and one… The water paled. Victory filled him. His lungs were beginning to hurt. A few more strokes and he would be out. He was counting wildly; he said a hundred and fifteen, and then a long time later, a hundred and fifteen again. The water was jewel-green all around him. Then he saw, above his head, a crack running up through the rock. Sunlight was falling through it, showing the clean, dark rock of the tunnel, a single mussel shell, and darkness ahead.