Barely visible from the massive dirt mounds that shielded Fort McAllister from the guns of Union ironclads, four squarely spaced posts jut from the Ogeechee River along the salt marsh.

Signs warn passing boaters -- "Submerged Object" -- with no hint of the Civil War shipwreck that lies on the riverbed 30-feet below.

The CSS Nashville, the first ship commissioned by the Confederacy in 1861, sank off Fort McAllister near Savannah on Feb. 28, 1863, under fire from the ironclad USS Montauk.

The Nashville is the only Confederate blockade runner known to exist in Georgia waters. Though divers from 1960 to 1983 salvaged artifacts ranging from 10-foot engine rods to brass lanterns, the state this fall will start its first archaeological survey of the wreck with an eye toward preserving it.

"I've been trying to get this for 20 years," said Danny Brown, manager of Fort McAllister State Park. "She's been pilfered pretty much her entire underwater life. As for whatever's still in the wreck, historically and archaeologically speaking, she's still a treasure."

Built in 1853 as a passenger and mail carrier, the Nashville was not one of the Civil War's technological marvels on par with the Confederate submarine H.L. Hunley or the iron-armored USS Monitor.

But the Nashville made history when the Confederacy commissioned the 1,221-ton steamer as its first warship in 1861 and it became the first vessel to fly the Confederate flag in international waters.

The Nashville was later reassigned as a privateer, a mercenary ship given license to raid enemy cargo vessels, and as a blockade runner that outran Union ships into southern ports to deliver arms and export cotton.

In its less than two years of war service, the Nashville so frustrated the Union Navy that Rear Adm. Samuel Francis Du Pont, commander of the South Atlantic Blockading Squadron, declared it "a thorn in my flesh."

"Her escape . . . would absolutely have shaken stocks in Wall Street," Du Pont wrote the day after the Nashville sank.

Last month, the Georgia Historic Preservation Division won a $39,996 grant from the National Park Service for an underwater survey of the Nashville and other war relics along nine miles of the Ogeechee River.

It will be the first major project for underwater archaeologist Jason Burns, who previously helped recover the Hunley in South Carolina, since he joined the Georgia Department of Natural Resources this year.

Using boats with high-tech gear such as side-scan sonar, which uses acoustic waves to capture detailed images of submerged objects, Burns hopes to get the most accurate picture of the Nashville's condition since the last divers saw it up close 21 years ago.

"It's never been archaeologically evaluated before," said Burns, who plans to begin work within two months. "We're not exactly sure what we're gonna find. "

Fort McAllister was built to protect a backdoor route to the Savannah port, and its earthen fortifications proved strong enough to hold off Union ironclads pursuing the Nashville in January 1863.

But the Nashville -- by then renamed the Rattlesnake -- ran aground while retreating from an attempt to run the Union blockade a month later. Cannon fire from the Montauk burned the Confederate ship to the waterline.

Judy Wood, a U.S. Army Corps of Engineers archaeologist and maritime historian who has catalogued records of 1,100 shipwrecks in Georgia, said that if any deserve to be studied, it is the Nashville.

"We can conclusively say this is our one and only blockade runner. It probably is one of the most significant wrecks in Georgia," said Wood, who has researched shipwrecks in the state for more than 20 years.

One of the Nashville's most important features to naval historians is its side-levered steam engine -- an innovation that kept the machinery used to turn the ship's paddle wheels lying horizontally within its hull. Other engines had parts extending vertically through a ship's deck -- making them a vulnerable target for gunfire.

"There are very few steam engines from the Civil War period surviving anywhere in the world," said Kevin Foster, chief maritime historian for the National Park Service. "Of the side-levered engines from the Civil War period, the Nashville is the only one that I know of."

Burns said his six-month underwater survey will determine if more of the Nashville should be salvaged. Fort McAllister already has on display a number of pieces raised over the past four decades.

The state hired divers in 1960 to use dynamite to remove engine rods that posed a hazard to boats at low tide. A private expedition from 1978 to 1983 raised furnace doors, cannonballs, wrenches and other tools. Four years ago, a group of crabbers stumbled onto the ship's rudder.

Part of a 1,700-acre state park, Fort McAllister has had its dirt-mound fortifications, cannons and ammunition magazines on public display for 40 years. But to Brown, the park manager, that is only half the story.

The rest remains beneath the Ogeechee River.

"The Union fleet never knew we were here until they chased the Nashville and bumped into us," Brown said. "So the Nashville and Fort McAllister are very much together in this."

Fort McAllister Park manager Danny Brown said the CSS Nashville is still a "treasure," despite the fact that the wreck has been pilfered for years. Park manager Danny Brown, left, and model craftsman Jim Goodwin with a model of the CSS Nashville near Richmond Hill, Ga.