JOHN YANG: Reefs are vibrant underwater ecosystems teeming with life.

Artificial human made reefs have been deployed around the country to enhance and protect coastlines.

According to one recent study, the area they occupy on the sea floor is the equivalent of 3,600American football fields.

And as Ali Rogin reports, what some of them are made of might surprise you.

ALI ROGIN (voice-over): Half a century ago, the SS United States set a record when it became the fastest ocean liner to cross the Atlantic Ocean.

Now it may set another record, becoming the world's largest artificial reef.

In February, the historic ship departed from its dock in Philadelphia, where it had been retired, and set sail to Mobile, Alabama.

Arriving in early March.

There, the ship will be stripped of chemicals, oil and other hazardous materials before eventually being sunk in the deep waters off the coast of Florida's Panhandle.

The aim of the project is to build tourism and create a new habitat for marine wildlife.

But artificial reefs, which can be made out of cinder blocks, lighthouses, and tires, in addition to ships, can also protect coastlines from storms by weakening waves crashing into shore.

That's the purpose of the U.S. Department of defense's re fence project, which is working to deploy artificial reefs in the waters off its coastal military bases.

Catherine Campbell manages the program.

Catherine, thank you so much for being here.

Why is the military looking into artificial reefs?

CATHERINE CAMPBELL, Defense Advanced Research Projects Agency: The military is looking into artificial reefs as a solution to protecting our coastal infrastructure.

And so we're looking for sustainable solutions that can protect the over 1700 bases and coastal installations that the DoD manages.

ALI ROGIN: So give me an example of a place where an artificial reef like this could have made a difference in terms of some of the damage that was later wrought to the military installation.

CATHERINE CAMPBELL: So, in 2018, Hurricane Michael swept through the panhandle of Florida and pretty much wiped most of Tyndall Air force base off of the map.

Almost every building on the base was destroyed.

It's a more than $5 billion project to rebuild this.

And so we're hoping that things like reef ENS can help protect our shorelines from storm damage like this.

ALI ROGIN: And how does that work?

How do these artificial reefs actually protect the shoreline?

CATHERIN CAMPBELL: The reefs that we are designing in this program are designed.

They're made of concrete.

But unlike a traditional breakwater, which is a solid piece of concrete, these are actually very porous structures.

They have lots of holes in them.

And so it's designed that when a wave comes up to these structures, all the energy is dissipated through them, and then you get calm water behind these reef structures.

So when you have a solid piece of concrete, like a traditional breakwater, the wave comes up and hits it and then reflects back out to sea.

It can cause all kinds of sediment, erosion, damage, and so it really doesn't protect the shoreline as well as some of these new structures can.

ALI ROGIN: Fascinating.

And why is it that this is such a useful application for military purposes?

CATHERINE CAMPBELL: So the other reason we're doing this is really to make something that is self-sustaining.

So the concrete pieces of this are just one part of our structures.

We also have an organic part, a biological part that is part of this system.

And so depending on where these structures are put in temperate waters we're going to use oysters, and in tropical waters we will use corals.

Those organisms will grow on top of these porous structures and then over time they'll become an actual natural reef which will not need the maintenance that we see in traditional breakwaters either.

That's the intention of this program so that you get a self-sustaining reef system.

And we know from data collected around the world that natural coral reefs do a very good job at protecting coastlines.

And so this is just a way to jumpstart that process and put these reef systems in where they might not otherwise be.

ALI ROGIN: And you recently installed one of these test projects.

How did that go and what did you learn from it?

CATHERINE CAMPBELL: Yeah, so we installed a test reef and these are still big projects.

So the test reef is 50 meters long.

It was installed off of Tyndall Air Force Base in November.

So we've seen some good storms come through this winter.

We've gotten a lot of good data to show that really these are working exactly as intended.

ALI ROGIN: What about the potential for civilian applications of this exact technology that you're piloting here?

CATHERINE CAMPBELL: Yeah, absolutely.

There is no reason that these could not be adopted for civilian infrastructure protection.

Part of our program is also looking at the techno economic analyses of these projects.

So, you know, do they compare with putting in traditional breakwater?

It's hard to put things in if they cost more than that.

So we're really pushing our performers to make sure these are comparable in their installation costs and then much lower in their maintenance costs.

So they become very attractive to putting in.

ALI ROGIN: Catherine Campbell, program manager for DARPA.

Thank you so much for joining us.

CATHERINE CAMPBELL: Thank you so much for having me.