The Mouth of Truth is the opening to the 100 ft. drop of Deep Hole. The area is surrounded by Pacific White-Sided Dolphins, an Ocean Sunfish (replaced by Apollo at night), and Japanese Eagle Rays. Enter the Mouth, and you'll find one of the two Giant Groupers that make their home here and a shoal of Pineconefish. The two smaller caves are Giant's Grotto and Serpent's Lair, which contain the other Giant Grouper and a Ribbon Moray, respectively. The entrance to Deep Dome is below the Mouth.
In the main storyline, Oceana is trapped here for a short while by an aggressive Tiger Shark after venturing to Deep Hole alone. After you calm the shark using the Pulsar, it goes away and only returns to circle the area at night.
This area's in-game description states:
"This large hole may be the feature that gave Deep Hole its name. It appears to be a mouthlike opening to another area."