MacKinnon and Liu have a boatload of questions about the sounds that help vessels navigate safely through the water. They want to know:

Where are these foghorns? How many of them are there? Why do we still use them despite technologies like radar and GPS? Who or what turns them on?

Let's find some answers.

Where are the foghorns and how many are there?

We start on Yerba Buena Island, right in the middle of the Bay Bridge, with U.S. Coast Guard Petty Officer John Sherwood of the Aids to Navigation Team San Francisco. One aspect of his job is maintaining foghorns, lights and other signals that are essentially the road signs of the water.

Sherwood shows me a map listing these aids to navigation -- things like buoys, signs and foghorns. Sherwood counts 11 foghorns that the Coast Guard operates in the bay, and roughly 20 more operated by other entities.

"We have horns located at Point Bonita, Yerba Buena Island, Alcatraz Island, Angel Island," he says. They're not only on islands, but also at bridges and lighthouses.

Each foghorn has a distinct sound and plays at a different interval, which is intentional. Sherwood says that in low visibility it's possible to figure out which point of land is closest if you learn the sounds of each foghorn well enough.

Sherwood also shows me around a Coast Guard parking lot. It looks like an island of misfit toys, with several 20-foot-long metal buoys on their sides.

Sherwood calls this area a "buoy depot." Here, the Coast Guard repairs buoys to put them back out in the water. The buoys are important instruments in the orchestra of the sea. Foghorns make music, but buoys do as well.

Sherwood says some buoys have huge bells that sound off when swayed by rough waves. Others have gongs -- made of stacks of metal plates. And there are also whistles.

Sherwood says that if we think we've never heard these before, it's more likely that we don't recognize them or know to listen for them. Some of these noises could be making their way to MacKinnon's and Liu's apartment, too.

Why do we still use foghorns if we now have sophisticated navigational systems?

To answer this one, I go to Vallejo, right by the Carquinez Bridge, to the California State University Maritime Academy. It's an institution that teaches students the ways of the sea.

Harry Bolton is the captain of the United States Training Ship Golden Bear. He shows me two large computers in the ship’s bridge, which is like a command center.

Bolton says the ship's radar and electronic navigation systems use the latest technology. But not all boats are equipped with it. Smaller vessels especially may still need foghorns to guide them.

"We sound the fog signals, so that if they don’t have this equipment they can avoid hitting us," Bolton says.

He says all these sounds, including the simpler technologies, are redundant, and together they make the bay a safer place.

So when it seems to MacKinnon and Liu that a foghorn is like a moving target, it’s because it might actually be a horn coming from a ship.

Who or what turns on the foghorns?

These days, a lot of foghorns are automatic. Angel Island, for example, has a sensor that detects particulates in the air. When there are enough particles, the foghorn goes off.

But some foghorns, even some very important ones, require a human to flip a switch or press a button.

This brings us to our final stop: the Golden Gate Bridge. Our guide is operating engineer Mario Territo with the Golden Gate Bridge, Highway & Transportation District.

When the bridge opened in the 1930s, he says, there were a lot of complaints from people who lived in San Francisco because they were being kept awake by the foghorns.

"They run 149 decibels," Territo says. That's nine decibels louder than the point at which a person starts to feel pain.

So, Territo says they redirected the foghorns more toward the center of the bay.

"And then the Alcatraz prisoners were complaining that it was interrupting their sleep," he says. Those complaints were largely ignored, however, and the foghorns stayed that way.

I meet Territo with the goal of seeing the Golden Gate Bridge foghorns. They're frequent favorites with some nautical experts because of their design: The horns run on compressed air, while more modern ones are electric. I also want to get a sense of just how loud these are, and find out who turns them on.

To get to two out of the five foghorns on the bridge, we have to go down 240 feet through a secret door that, if you’ve ever walked across the Golden Gate Bridge, you have passed and likely not noticed. Then we squeeze into a phone booth-size elevator inside the South Tower of the bridge.

When we scramble out onto a concrete platform, we’re just 40 feet above the bay waves.

“As you can see, we’re on the South Tower Pier,” Territo says.

He points to two orange tanks on the concrete platform. They are each about the size of a four-door car.

“We had an original air tank for the foghorns that was put in here in 1937 and it rusted out about a year ago," he says. They've replaced both.

The foghorns work by pushing compressed air through the noisemakers. This air is pumped to the tanks from a building right by the bridge toll booths.

At one end of the platform where we stand, there, in all of its international-orange glory, is a 4-foot-long foghorn.

We shove neon earplugs into our ears. Territo radios engineers to blow the foghorns. The way they operate something very old is through something very new: a touch-screen monitor. While it’s not foggy in the slightest, they do it. For me. For you. For Andy MacKinnon and Jen Liu.

Territo summarizes the feeling best.