For decades after the disaster, there was little doubt about what sank the Titanic. When the "unsinkable" ship, the largest, most luxurious ocean liner of its time, crashed into an iceberg on its maiden voyage in 1912, it took more than 1,500 of its 2,200 passengers to the bottom. As the ship slipped into the North Atlantic, so, too, did the secret of how and why it sank.

[See photos commemorating the 100th anniversary of the sinking of the Titanic.]

Two government investigations conducted immediately after the disaster agreed it was the iceberg, not any weakness in the ship itself, that caused the Titanic to sink. Both inquiries concluded the vessel had gone to the bottom intact. Blame for the incident fell on the ship's deceased captain, E. J. Smith, who was condemned for racing at 22 knots through a known ice field in the dark waters off the coast of Newfoundland. The case of the Titanic was considered closed.

But lingering questions about what might have sunk the seemingly indestructible ship never completely disappeared. In 1985, when oceanographer Robert Ballard, after years of searching, finally located the ship's remains 2.5 miles down on the ocean bottom, he discovered that it had, in fact, broken in two on the surface before sinking. His findings made the Titanic rise again in the public imagination. Why had it cracked, experts wondered? If the official inquiries were wrong, was the invincible Titanic weak? A few years after Ballard discovered the wreck, the first pieces of the ship were brought to the surface, raising even more eyebrows when they seemed to offer physical evidence that low-quality steel might have caused the disaster. In 1997, James Cameron's film Titanic, largely mirroring the scientific consensus at the time, seared Titanic's terrifying last moments, with its stern soaring high into the air before it cracked in two and disappeared, into popular memory.

Still, the search for answers about the Titanic didn't end there. In two new books, a group of historians, naval architects, and materials scientists argue that fresh evidence has further unraveled the familiar story of the Titanic, raising more questions about what caused the disaster. In What Really Sank the Titanic: New Forensic Discoveries, Jennifer Hooper McCarty, a materials scientist at Oregon Health and Science University, and Tim Foecke, a scientist at the National Institute of Standards and Technology, make the case that it wasn't the ship's steel that was weak; it was the rivets, the all-important metal pins that held the steel hull plates together. Titanic's Last Secrets, to be published next month, describes the work of Richie Kohler and John Chatterton, wreck-diving historians who believe two recently discovered pieces of the Titanic's bottom prove the ship's stern never rose high in the air the way many Titanic experts, including Cameron, originally believed. The two divers, whose discovery of a lost German U-boat was chronicled in the book Shadow Divers, say the ship broke up and sank while still relatively flat on the surface—a potential sign of weakness, they believe, that was covered up after the disaster.

When the Titanic's keel was laid down in 1909, Harland & Wolff, the Belfast shipbuilder that constructed the ship, certainly didn't believe its design would still be controversial a hundred years later. Built in response to a rival company's construction of a new generation of fast liners, Titanic and her sister ships, Olympic and Britannic, were the biggest ships ever made—from bow to stern, they were almost 900 feet long, dwarfing even the world's biggest skyscrapers. Specially outfitted to handle the challenges of the North Atlantic, including big waves and major collisions, they were also supposed to be among the safest. The Titanic could stay afloat with four of its 16 watertight compartments flooded, more than anyone could imagine on a ship of its size.

On the night of April 14, 1912, though, only a few days into the Titanic's maiden voyage, its Achilles' heel was exposed. The ship wasn't nimble enough to avoid an iceberg that lookouts spotted (the only way to detect icebergs at the time) at the last minute in the darkness. As the ice bumped along its starboard side, it punched holes in the ship's steel plates, flooding six compartments. In a little over two hours, the Titanic filled with water and sank.

Low quality. More than 70 years passed before scientists were able to study the first physical evidence of the wreck. As luck would have it, the first piece of steel pulled up from the bottom seemed to put an end to the mystery. When the steel was placed in ice water and hit with a hammer, it shattered. For much of the 1990s, scientists thought this "brittle" steel was responsible for the massive flooding. Only recently has testing on other, bigger pieces of the ship disproved this theory. The original piece, scientists discovered, had been unusually weak, while the rest of Titanic's steel passed the tests. "We know now there was nothing wrong with the steel," says William Garzke, chairman of a forensics panel formed by the Society of Naval Architects and Marine Engineers to investigate the wreck.

Experts looking for explanations landed on another potentially weak link: The more than 3 million rivets holding the ship together. McCarty and Foecke began examining 48 rivets brought up from the wreck and found they contained high concentrations of "slag," a residue of smelting that can make metal fracture prone. Researching in the Harland & Wolff archives, they discovered that the shipbuilder's ambitious plans to build three large ships at the same time had put a huge strain on its shipyard. "Not because of cost, but because of time pressures, they started using lower-quality material to fill the gaps," says Foecke. This substandard iron was pounded by hand into the ship's bow and stern, where the large machines required to pound in steel rivets didn't fit. Steel rivets, meanwhile, which are much stronger than iron, were put in the more-accessible middle of the ship.

When the Titanic hit the iceberg, McCarty and Foecke say, the weaker iron rivets in the bow popped, opening seams in the hull—and hurrying the ship's demise. It's no accident, Foecke says, that the flooding stopped at the point in the hull where the steel rivets began.

Harland & Wolff, now an engineering and design firm, flatly rejects the notion that its rivets were weak. Tom McCluskie, the company's retired archivist, points out that Olympic, Titanic's sister ship, was riveted with the same iron and served without incident for 25 years, surviving several major collisions, including being rammed by a British cruiser. "Olympic deliberately rammed a German submarine during the First World War and cut it in half," says McCluskie. "She was plenty strong." The Britannic sank after hitting a mine during World War I. Both ships were strengthened after the Titanic disaster with double hulls and taller bulkheads, but their rivets were never changed.

Stronger rivets might have slowed the sinking process, but once water began flooding six of the Titanic's compartments, it was only a matter of time before the ship went down. Questions remain, though, about exactly how and why the ship ultimately broke apart and sank. In 2005, an expedition organized by Kohler and Chatterton found a new clue. Wandering away from the main wreckage site, they stumbled upon two large pieces of the ship's bottom on the ocean floor. Closer examination revealed the two hull sections had split exactly where the ship broke in two, making them a possible key to the mystery of the ship's final moments. Simon Mills, an Olympic-class-ship historian who advised the divers, calls the find "very likely the most interesting piece of Titanic research to be carried out in the last 20 years."

When Roger Long, a naval architect hired to accompany the expedition, began analyzing the edges of the hull pieces, he came to a surprising conclusion. It was impossible, he believed, for the ship to have broken up the way experts for two decades believed it did, with the stern rising up to a 45-degree angle before the ship's hull split. "There are a lot of very contradictory things you can see in the pieces," he says. "But the only scenario I could come up with to explain all of the contradictions was that the ship broke at a very shallow angle." Close examination of the pieces showed that they had been interrupted in the middle of tearing apart—a sign, Long says, that the ship was still at a low-enough angle (he estimates only 11 degrees) that its stern could regain buoyancy as it began to crack. If the back of the ship had been raised out of the water at a 45-degree angle, as depicted in Cameron's movie, once the stern tore off, nothing would have stopped it, and the hull pieces would have torn in two.

Why does it matter exactly how the ship broke in two? For Titanic's passengers, it may have been the difference between life and death. "In the movie, the stern rises up and [then] sinks," says Chatterton. "It's this protracted, dramatic experience." But in Long's scenario, the ship may have tilted over only slightly as the bow filled with water, giving those on board a false sense of security. "If you're standing on the deck with 10 degrees of incline, and they're saying 'Quick, everyone into the lifeboats,' you're thinking, 'You know, things aren't looking so bad here, maybe I can just stay in the bar,' " says Chatterton. "The passengers and many of the crew didn't understand the seriousness of the situation they were in." Of course, since the Titanic had enough lifeboats for only half its passengers, many people were never going to make it off the ship alive. When the bow filled with enough water, Long says, the ship split in two and sank in a matter of minutes.

Interestingly, much of the survivor testimony seems to confirm this sequence of events. Charlie Joughin, Titanic's chief baker, said that he had been standing near the stern when the ship went under, but he reported none of the signs of a high-angle break. No suction, no big splash, and no roller-coaster ride to the surface. He said he swam away from the ship without even getting his hair wet. Unlike in the Cameron film, there was no huge wave reported from any of the lifeboats when the stern went under. One survivor reported slipping into the water, turning around, and discovering the ship had disappeared. "He was in the water 50 feet from the ship, he heard a 'shloop,' and it was gone," says Long. "That's not what a person would remember if 25,000 tons of steel fell nearby."

Eyewitnesses. While some survivors in the lifeboats did remember seeing the ship's stern rising high in the air, Long says that might have been an optical illusion. At an 11-degree angle, the ship's propellers would have been raised out of the water, making the ship, already nearly 20 stories tall, appear even taller and making its angle in the water appear even steeper. Technical advisers to the movie Titanic say Cameron, who did not respond to a request for comment, may have been aware of this but exaggerated the angle at which the ship sank for effect.

Though experts still quibble about the exact nature of how the ship broke up, a consensus does seem to be forming around how Titanic sank. "We all agree that the ship did sink at a shallow angle," says Garzke, head of the naval architects' forensics panel. Historians believe Harland & Wolff was probably aware of this at the time, but when the official inquiries absolved the shipbuilder of any liability in the matter, the company didn't protest.

Some conspiracy theorists believe that the company's silence was a sign of a coverup, and that the post-disaster retrofitting of Titanic's sister ships proves Harland & Wolff knew its ship was flawed. But most historians come to a different conclusion. "The fact that the ship broke up on the surface does not mean she was weak," says Long. When 38,000 tons of water filled its bow, pushing the stern up even 11 degrees out of the water, the ship was loaded beyond its capacity and cracked in two.