BAY HARBOR ISLANDS, Fla. -

The Broad Causeway is the primary bridge connecting the Town of Bay Harbor Islands with the mainland. For more than six decades, this landmark structure has been under constant assault by the Intracoastal waterway -- pounding waves, salty air, and relentless tides.

More than 21,000 cars a day cross over the Broad Causeway, despite what's happening under it.

We set out to see why the Department of Transportation considers this bridge "structurally deficient."

SPECIAL SECTION: Bridges on the Brink

Since bridges are a matter of national security, that information is classified. But documents provided to Local 10 show that in January, state inspectors lowered the causeway's Health Index number to 10 out of 100. It's not hard to see why.

"It would be a major, major problem if this bridge were to ever be posted or shut down," said Town Manager Ronald Wasson.

Wasson says Bay Harbor Islands spent nearly $3 million just six years ago, repairing the aging structure -- trying to prolong it's life -- but the corrosion continues.

The substructure of the bridge is riddled with rust. It's spreading like cancer.

In some places, the concrete on the pilings -- the columns that hold up the roadway -- is cracked and falling apart, exposing the corroded rebar inside.

Underwater, layers of barnacles hide the damage.

"It's brutal, a brutal environment" Wasson said.

Wasson says that early next year, the Broad Causeway will get a major makeover. The cracked concrete will be replaced, the system of fenders at the waterline rebuilt, the decking and electrical components of the drawbridge will be all new. And the entire span will be fitted with an electrolysis system to protect the new rebar from corrosion.

"This is a major overhaul for this bridge," Wasson said.

Wasson estimates it will cost $8 million to $12 million to repair the Broad Causeway. The town will borrow money to pay for most of it. The rest will come from tolls.

"When people come into Bay Harbor Islands and they come across the Causeway, I think it's one of the most beautiful 60 seconds that you're going to ride into town," Wasson said.

The Broad Causeway is more than just "the way home" for residents. I's a gateway to their town. The man who built it died at the age of 95. City leaders hope his bridge will last as long.

"The bridge is very well engineered and very well designed and we're going to make sure the life of this bridge goes another 20 to 30 years," Wasson said.