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production supposedly caused the ship to crumple like a piñata.
    The bad-steel theory emerged from the study of a hull piece recovered in 1991. Canadian metallurgists concluded that the steel had shattered. They based their conclusions on findings of high sulfur and phosphorus content; a mechanical impact test; and a photograph of the edges of a hole punched in
Titanic
’s sister ship
Olympic
.
    Research on steel samples retrieved in 1996 and 1998, performed by scientists at the University of Missouri-Rolla, laboratories at Bethlehem Steel, and other institutions, rebutted those findings. It declared
Titanic’
s hull stronger than first reported, and about as strong as could be hoped for given the industry of the time.
    The next theory to gain wide attention focused not on the steel plates, but rather the rivets that held them together. A 1996 expedition brought up a hull plate bearing holes left by popped rivet heads. In addition, 17 of 18 iron rivets brought up from the wreck lacked at least one head, suggesting they had broken under stress.
    Forensic investigators suggested in a 2008 book that the iceberg broke the inferior rivets. They pointed to the likelihood of improperly driven, low-grade rivets, especially in the hand-hammered bow section. To meet its construction deadline, Harland & Wolff had added riveters with limited experience and pushed them to hurry.
    P. H. Nargeolet, a scientist who has made many trips to
Titanic
, scoffs at the idea. “They knew how to drive rivets,” he said. “I don’t think they did a bad job, and they used the best-quality steel on the market. Most of the rivets are still there.”
    A third theory suggested that
Titanic’
s structure suffered from overall weakness due to unexpected engineering problems. One line of inquiry centered on
Titanic’
s twin,
Olympic
. If
Olympic
suffered structural weakness,
Titanic
likely did too.
    Off the Irish coast in 1911, Harland & Wolff chief designer Thomas Andrews stepped onto
Olympic’
s bridge wing during a sea test and felt the deck ripple. Curious, Andrews stuck his head over theside rail.
Olympic
gasped like a greyhound, its hull moving three inches in and out. “Sides panting at cruising speed,” he jotted in his notebook.
    Big ships conceived during that era presented big design questions never raised before, such as: What’s the minimum steel strength needed for a ship more than 882 feet long? Andrews had asked for hull plates 1¼ inches thick, but Harland & Wolff insisted on just 1 inch to make
Titanic
go faster. In response to
Olympic’
s trials, Andrews reinforced
Titanic
with steel bracing in its bow and at the juncture of hull and double bottom. He enclosed the Promenade Deck. Modern investigators wondered if it was enough, prompting a second line of inquiry: Did more changes in ship design follow the
Titanic
disaster?
    Divers to
Britannic
,
Titanic’
s second sister (originally named
Gigantic
), which sank in World War I near Greece, discovered expansion joint upgrades evidently put in place after
Titanic
sank. Expansion joints reduce stress by letting components of large structures move. In sidewalks and bridges, such joints are built-in gaps that accommodate expansion and contraction in heat and cold; in large ships they exist as narrow, open spaces in the upper portions of the hull and decks that allow them to flex atop ocean waves without cracking,
    Naval architect Roger Long examined images of steel torn from the hull near
Titanic’
s aft expansion joint, and added data about
Britannic
‘s redesign. He concluded in 2005 that despite Andrews’s improvements,
Titanic’
s hull remained weak, especially at the expansion joints. Skeptics pointed out expansion joints extended only through the top two decks, limiting their structural impact. Rather than the single key to
Titanic’
s demise, the joints might have just been one of many—the straw that