"Ahab could discover no sign in the sea. But suddenly as he peered down and down into its depths, he profoundly saw a white living spot no bigger than a white weasel, with wonderful celerity uprising, and magnifying as it rose, till it turned, and then there were plainly revealed two long crooked rows of white, glistening teeth, floating up from the undiscoverable bottom. It was Moby Dick's open mouth and scrolled jaw; his vast, shadowed bulk still half blending with the blue of the sea. The glittering mouth yawned beneath the boat like an open-doored marble tomb; and giving one sidelong sweep with his steering oar, Ahab whirled the craft aside from this tremendous apparition."
Musing: Here is the sighting of Moby-Dick at last. Ahab is in his whale-boat with his crew of stowaways. Though the boat had already given chase, the sailors lost sight of Moby-Dick. Ahab is the one to spot Moby-Dick under the water. Ahab recognizes the teeth in the whale's jaw - the teeth that ripped his leg from his body. There is foreshadowing in the door like an tomb. There is also no going back from this point on.