In the busy harbour of the Port of Miami, a diver is crawling on the seabed at 12m, about to disappear under a 65,000 tonne cruise ship into a crawl-space less than 1m high beneath its long keel and the bottom.
If the ship should rock, even gently at anchor, the man could be crushed. He is using scuba gear in blinding currents of swirling sand and dangerous bottom debris. And he has to avoid powerful hot and cold intakes and exhausts that could trap or drown him.
Above him are 12 decks of ship holding 4000 passengers and crew-a perfect terrorist target. A successful attack at a busy centre like this could destroy sea-going tourism for years, shut down hotels and devastate the area's economy.
The underwater man is searching for a bomb. He's a police diver engaged in one of the most important, dangerous and until now secretive jobs of our time.
The Port of Miami is a gateway to America. Into its harbour every year comes $12 billion dollars' worth of business on cruise ships, oil tankers and freighters connecting 240 countries, moving almost 4 million passengers and carrying more than a million cargo containers. Every day passengers from all over the world pass through the port. On board they dance, eat, gamble and swim, oblivious to all the measures being taken to keep them safe.
While 97,000 people have port-related jobs in Miami, it's down to five police divers and a unit leader to regularly check the hulls of the 800-plus cruise liners and 2000 cargo ships entering and leaving each year.
They train with veteran military divers who provide classified briefings on explosive weapons, and attach training dummies to hulls. Under water, arrayed in a long harness of lines, the searchers work as a team, looking for explosives or drugs attached to ship hulls.
In their conversations, they never use the panic-causing word "bomb". They prefer the word "device".
Police diver Luis Sierra, 34, is the father of two girls and awaiting the birth of a son. "Devices" are not the only danger he faces. "I often fight disorientation," he says. "Which way is up? I have to turn on the small light above my mask and look for my exhaust bubbles floating upwards. To find 'down', I sometimes just push my hands into the cold muck at the bottom of the harbour." The divers are of course well-trained for working in near-zero visibility conditions.
"One day a ship came in from Jamaica with a welded metal box attached to a stabiliser far down on her hull," says Sierra. "We wanted to wait to see if anyone would come to retrieve it but were ordered by federal authorities to bring it up so it could be opened immediately. The box contained 120 pounds [55kg] of compressed marijuana."
The dangers of the job don't end on dry land. Three divers working for Jamaican police were kidnapped and murdered by Kingston drug lords in 2001. As a warning to others, one was fed alive and screaming through a tree-chipper.
Sierra and the team remain unfazed. "We're police officers first and then divers. We'll be over to Jamaica in a heartbeat if they call on us for help." But despite being expert marksmen, the divers are not armed when submerged. "We're not in an action movie," says Sierra. "You don't want to be under water in a life and death struggle."
Until recently, the team has been a guarded secret. "Our work was always unknown to the public, but now it is official policy to let terrorists know we are on the job," says Sierra.
While the FBI has been checking non-American students at flying schools, other have been working with dive shops, quietly investigating people taking diving lessons or placing orders for specialised gear such as rebreathers.
Recently all ports in the USA were warned by the FBI about the possible threat of underwater terrorists. The information apparently emerged from the Guantanamo Bay detention camp in Cuba.
Agents are now seeking the names of anyone in America trained in the use of rebreathers. "If, say, traffickers came to retrieve drugs attached to a ship's hull, we would watch for exhaust bubbles if they were using scuba tanks. But they could come unnoticed- terrorists usually have unlimited expense accounts, and they could afford the best rebreather units!" says Sierra.
Security is now a top maritime priority. As well as police and coastguard protection, most ships drop their own lifeboat tender, manned 24 hours a day, to watch for any dangers in the vicinity.
"We haven't had the pleasure of checking the Queen Mary 2, which comes into Port Everglades at Fort Lauderdale," says Luis Sierra. The giant ship has an array of small floating docks all around it to prevent any sort of suicide attack by water. It also has its own large security force and probably its own divers.
"Warships also have their own divers," explains Sierra. "The US Navy has electronic surveillance systems and human watchers looking after its ships. But we do searches of the seawalls where they will dock- it's our job to provide them with a safe parking lot.
"The Navy has hi-tech underwater communications systems for its divers. But we use 'line-of-sight calling', with face-masks that broadcast to one another."
Checking ships is complex. Police divers must co-ordinate their dives with shipping traffic, tides and time.
Only those crew-members who need to know are warned that divers are coming. A boarding team introduces itself to the captain and engineer, then goes through a US Coastguard-approved "tag off" procedure. Red tags are placed and initialled over every control that could turn on or off machinery that could harm the divers. "If someone turns on the steering gear while we're checking it, we could be crushed," says Sierra. "A thruster could wash us out or suck us in. A diver could be sucked up against a grille and die, just like victims in large swimming pools.
"We must be careful using RF [radio frequency] broadcasting when doing explosive searches. And I'd rather have no light, because of photo-sensitive detonators on some devices that we've learned about. I don't want to be the one who triggers anything.
"If we do find a 'parasitic device', we can either videotape it with a portable underwater camera or provide a live feed to expert analysts on shore, showing what we see. If it's confirmed, the ship would immediately be evacuated and US Navy and FBI ordnance divers would be called in to disarm and dismantle it."
Luis Sierra is currently taking his Masters degree in Criminal Justice. He tells me that his hobby "used to be diving- but I lost it when I came to this unit. Now my hobby is being the best husband and father that I can be."
Working in the luxury of high tide conditions, one of the divers searches the bottom of a cruise ship for explosive devices or drugs.
Police divers Luis Sierra (left) and Oscar Rogue are part of the team of six that inspects the hulls of vessels in the Port of Miami.
The nitrox-equipped team heads out toward ships arriving in the port, ready to check the hulls for bombs
One mistakenly turned-on propeller or intake system could kill divers already working in taxing conditions.
Police divers go down the anchor chain of a giant cruise liner