Philadelphia — Up on deck, where the casket of the unknown soldier was tied down with rope and covered with canvas, the Marine guards lashed themselves to the ship’s stanchions so they wouldn’t be swept overboard.

Twenty-foot seas broke over the pilot house. One Marine was drenched by a wave so big that it tore his hip boots off. And the ship was rolling so badly that the crew feared that each roll would be its last.

It was the fall of 1921. The USS Olympia was halfway across the stormy Atlantic, bound from France to the Washington Navy Yard. And the Marine commander realized that if the hallowed casket went over the side, he might as well go, too.

The captain asked the chaplain to pray.

As the country this year marks the 100th anniversary of the Tomb of the Unknowns in Arlington National Cemetery, a museum here plans to tell the little-known story of how the ship and its cargo were nearly lost at sea.

The Independence Seaport Museum, adjacent to where the 129-year-old Olympia is docked on the Delaware River, plans to open on May 28 an exhibit titled “Difficult Journey Home,” organizers said.

It will recount how the illustrious old ship battled alone through mountainous seas and the remnants of two hurricanes to save itself and the unknown soldier, and avoid a national catastrophe.

The now-famous Tomb of the Unknowns was established in 1921 to honor the 117,000 Americans killed during World War I. Following the practice of Britain and France, U.S. officials selected the remains of one unidentified soldier to be buried in the tomb.

Amid solemn fanfare, the unknown was selected from a cemetery in France and his remains put aboard Olympia and brought to Washington. It was placed in the tomb in Arlington, across the Potomac River from Washington, on Nov. 11, 1921, three years after the war ended.

Unknown soldiers from World War II, the Korean War and the Vietnam War were added later, though the Vietnam unknown was subsequently disinterred and identified.

The hilltop site has become one of the country’s most sacred memorials, with quiet crowds gathering to watch the hourly changing of the guard. But the dramatic journey of the first unknown aboard Olympia a century ago has rarely been told, said the museum′s chief curator, Craig Bruns. And few realize how close it came to disaster.

In “all the history books … the unknown soldier is selected [in France], Olympia picks up the body, and the next paragraph is Olympia drops off the body in Washington,” Bruns said in a recent interview.

The dicey ocean crossing is the missing chapter.

“Why was the story not recorded?” he said. “Was it seen as distracting from the unknown soldier” story? Was it embarrassing to the Navy?”

In 1921, Olympia was probably the Navy’s most famous ship afloat — earning its laurels during the brief and lopsided Spanish-American War of 1898.

But it was almost 30 years old. It was still fitted with a ram in its prow and was dwarfed by the more modern battleships that emerged during World War I, according to historical accounts.

Although it had steamed over many seas, it was known as “a roller,” and during the journey with the unknown soldier it reportedly rolled near to the point of capsizing, Bruns said.

For most of the voyage, the ship was traveling alone and fighting turbulent “cross seas,” where the waves come from different directions.

And despite the prospect of bad weather, the unknown’s casket was carried up on the weather deck, possibly because it was too big to fit through the narrow hatches to be sheltered below, Bruns said.

“It seems odd that the Navy would have knowingly allowed this precious cargo to be left out in the weather,” he said.

Did officers realize belatedly that the casket wouldn’t fit “and they couldn’t cram it through” without jostling the remains inside? he wondered.

Did they know all along and shelter it on deck as well as they could?

“I don’t have the answer,” Bruns said.

Ship’s carpenters eventually built a wooden box to hold the casket. It was covered in canvas and tied down between two tall ventilation funnels with enough rope to secure a battleship, as one Marine put it.

“We lashed this fellow down with everything we could tie on him,” recalled then 24-year-old Marine Capt. Graves B. Erskine.

The selection of the American unknown took place on Oct. 24, 1921, in the city of Châlons-sur-Marne, now Châlons-en-Champagne, about 100 miles east of Paris.

The bodies of four unknowns were exhumed from four cemeteries, placed in caskets and brought to the town hall, according to a history of the Graves Registration Service that is held at the National Archives.

One body was selected at random by an Army sergeant, Edward F. Younger, a combat veteran stationed in Europe who had been picked for the duty. Younger made the selection by placing a bouquet of roses on one of the caskets.

The body was then taken by special train to Paris and on to the port of Le Havre, where Olympia was waiting, its flags at half-mast.

On Oct. 25, as the casket was carried on board, the ship’s band played the Star Spangled Banner and the French national anthem, “La Marseillaise.”

Following solemn shipboard ceremonies, Olympia slowly began its journey home.

As it headed out into the Atlantic, two storms were roiling the ocean ahead.

The first, a tropical storm that became a hurricane, had cut across Florida in mid-October, heading east into the Atlantic and making a U-turn across Olympia’s path on Oct. 22.

The second storm had roared through the Gulf of Mexico and slammed Tampa Bay, Fla., before crossing the state and heading in a weakened state on a course that would intercept the Olympia about Nov. 3, according to the museum.

Two days out of Le Havre, the weather turned sour. The ship’s log records heavy, chopping, cross seas on the afternoon of Oct. 27. Three days later, at 5:54 a.m., lookouts sighted Terceira Island, in the Azores, far out into the Atlantic.

The ocean was still disturbed, with long, rolling cross seas, according to the logs. The afternoon of Oct. 30, Olympia steamed through the channel between the volcanic islands of Faial and Pico in the Azores and out into the open Atlantic beyond.

It had 2,700 miles to go.

On Halloween morning, the weather deteriorated. Near gale-force winds out of the southeast drove waves the size of houses over the ship. “Heavy weather,” the log states.

One wave broke a life raft. Olympia staggered and slowed to two-thirds speed. “Unable to keep up steam,” the log says. Ankle-deep water sloshed below deck. And in the firerooms, begrimed sailors frantically shoveled coal into the boilers.

But it wasn’t enough. All off-duty sailors and Marines were ordered to set up a kind of bucket brigade, using wheel barrows to shuttle coal from other coal bunkers, according to a 1964 account by retired Marine Corps Lt. Col. Dennis D. Nicholson.

And lightened by heavy coal consumption, the Olympia pitched and rolled even more violently, Nicholson reported. A Marine private, Frederick A. Landry, remembered: “All hands and the ship’s cook speculated on how close she would come to capsizing on the next roll.”

Up on deck, conditions were terrible. The Marine guards assigned to stand watch over the unknown held on for their lives, and extra men stood by in case the container holding the casket tore loose.

Capt. Graves Blanchard Erskine, who had been severely wounded during the war and would become a general in World War II, was in command of the Marines on board. He saw the deck plates moving back and forth as if they might come apart.

“The bridge itself was awash half the time,” he recalled. “And almost as often there was seawater in the wardroom.”

“The agonizing thought came to me: what if the Unknown Soldier — the hero all America awaits to honor — is washed overboard?” he remembered. “I knew that if such a thing happened, I might as well jump over with him.”

The storm abated on Nov. 1. A rain squall passed. Olympia slowed to tighten some water seals. But the next day, the sea was rough again, most likely churned up by the remnants of the Tampa Bay storm as it passed ahead of Olympia.

The log recorded heavy seas. A sailor was injured when a water bucket fell on his head. Thirteen-foot waves pounded the ship, and winds howled out of the west at 30 mph for much of Nov. 5 and 6.

On Nov. 6, a man was injured when he lost his balance and fell on a cylinder head in the engine room.

As the ship was battered, the skipper, Capt. Henry Wyman, asked a chaplain, Navy Lt. Edward A. Duff, to gather those off duty and pray for Olympia’s survival. They assembled near the galley, and held on as the ship pitched and rolled.

Duff told the crew that God was watching over them, according to Nicholson’s account. He told them that the nation was waiting for them. And he prayed that the unknown soldier on the pitching deck topside would soon be safe.

At 9:56 a.m. on Nov. 7, Olympia’s crew sighted the lighthouse at Cape Henry, Va., at the mouth of the Chesapeake Bay. The ship and the men had brought the unknown home.

A group of Marines was allowed on deck for some fresh air. The ocean was still rough. But a Pvt. Landry remembered: “Never has there been a happier bunch of Marines.”

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