It's the nations busiest port complex, situated smack dab in the middle of Southern California. 7,500 acres of waterways, piers, rigs and barges, handling more than $150 billion in cargo and more than a million cruise ship passengers every year.

"Our mission is to ensure the safety and security of the port and our officers train for that. They have the mindset that we're combatting terrorism here and we're focused everyday on ensuring that we make the port as safe as it can possibly be," said Los Angeles Port Police Captain Mike Graychik. He is one of 15 men read to dive to the depths of these murky waters on a moments notice. Men who train year around and face some tough waters every day.

"Port operations go on 24-7 and diving in the port is dangerous diving, it's dark, it's dirty, there's a lot of structures in the harbor that are potentially dangerous," said Graychik.

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That means diving in water that can be tainted with bacteria, especially after rains dump millions of gallons of runoff into the harbor. Then there's the massive cargo ships, cruise ships, water taxi's and every day boating enthusiasts who turn the bluish, grubby sea lanes into a boating freeway of sorts.

The dive team watches it all, while combing ship hulls, pilings, pipelines and the channel looking for explosives or any other threats to national and port security and as officers tell us, "operationally, we're out here everyday. We've got eyes on the different activities that are going on, we interact with different entities out here in the maritime envionment, tugboat operators, the fishing boat operators, the longshoremen, we talk to the other eyes and ears out here."

The divers cover every inch of underwater infastructure, crucial to the operation of the port and to the economy of the entire western United States. It's a job that depends on close coordination with hundreds of different public agencies and commercial interests from around the globe. Part of the P-Dog team (Port Dive Operations Group), divers also get help and support from the United States Coast Guard, who oversee much of what goes on.

As Graychik looks out over the waters towards his teams dive boat he says, "We've got eyes on the different activities that are going on, we interact with different entities out here in the maritime envionment, tugboat operators, the fishing boat operators, the longshoremen, we talk to the other eyes and ears out here."

Eyes and ears that are fixated above the water while divers from the Los Angeles Port Police Department train and protect areas below the surface. So long as things go right, no one will ever know when they are down below and where they search for threats — and that's just the way they like it.

Adam Housley joined Fox News Channel (FNC) in 2001 and currently serves as a Los Angeles-based senior correspondent.