Jan. 20, 2012 — -- intro: Eleven people are dead and 21 remain missing a week after the cruise ship Costa Concordia collided with rocks and capsized Friday evening near the Italian island of Giglio. Families of those missing, including Minnesota couple Jerry and Barbara Heil, continue to hope that their loved ones will be found.
But it's not just maritime tragedies on the scale of the Costa Concordia that result in vacationers missing at sea. Long before last week's accident, dozens of men and women mysteriously vanished aboard cruise liners, confounding investigators and leaving their families heartbroken. A number of families have blamed cruise lines for failing to report disappearances in a timely fashion and not doing enough to ensure the safety of passengers. The cruise industry has maintained that accidents and crime aboard cruise ships are rare.
In the last two years, 41 people went overboard or went missing on cruise ships, according to CruiseJunkie.com, a cruise watchdog site. The cruise industry reported that nearly 15 million passengers traveled on cruise ships in 2010 alone.
"(C)ruising continues to be one of safest means of travel among all types of vacationing," the Cruise Lines International Association said in a statement released after the Costa Concordia tragedy.
In 2010, Congress passed a law to improve safety and accountability on cruise ships. The Cruise Vessel Security and Safety Act stipulated more surveillance aboard ships, crime response training for ship crew members and a structure to improve communication with the Coast Guard and the FBI, among other requirements.
Here are some of the most baffling cruise ship missing persons cases in recent years:
quicklist: 1category: title: Annette Mizenerurl: media: 15396777text: Annette Mizener, 37, of Waukesha, Wis., went missing Dec. 4, 2004, from the Carnival ship Pride off the coast of Mexico. She was on vacation with her parents and a 17-year-old daughter.
The ship's crew launched a room-by-room search of the Pride after Mizener's daughter Danielle and other passengers noticed she was missing. Mizener's purse was found near a railing on one of the ship's lower decks.
Coast Guard aircraft and a Navy ship joined in the 16-hour search for Mizener but turned up nothing.
Mizener's husband said at the time that he suspected his wife fell over the side of the ship, either by accident or as the result of foul play.
Watch the full story now on "Cruise Ship Confidential," a "20/20" special.
quicklist: 2category: title: George Smithurl: media: 15388809text: George Smith, 26, of Greenwich, Conn., disappeared July 5, 2005, from the Royal Caribbean cruise ship Brilliance of the Seas while traveling through the Aegean Sea between Greece and Turkey.
Smith was on his honeymoon and vanished after a night of heavy drinking with his new wife. Fellow passengers reported hearing a loud noise from his cabin the night of his disappearance.
Traces of blood found at the scene led investigators and family members to suspect foul play, but Smith's body was never found.
quicklist: 3category: title: Daniel Dipierourl: media: text: Daniel Dipiero, 21, of Canfield, Ohio went missing aboard Royal Caribbean's Mariner of the Seas on the first night of his cruise in May, 2006, as the ship headed to the Bahamas from Port Canaveral, Fla.
DiPiero was traveling with six friends, who say he left them around midnight on Sunday to return to their shared stateroom. Later, between 12:12 a.m. and 2:16 a.m., ship security cameras captured him standing along a railing and reclining in a chair on a side deck. With no evidence that he had ever slept in their cabin, the friends reported DiPiero missing at 11 a.m. on Monday.
Despite a Coast Guard search, DiPiero was never found.
quicklist: 4category: title: Merrian Carverurl: media: 15388825text: Merrian Carver, 40, of Cambridge, Mass., vanished aboard the Celebrity Cruises ship Mercury on the second night of a seven-day Alaskan cruise in August, 2004. The cruise line never reported Carver's disappearance to her family and donated the clothing she left behind to charity.
Carver's parents spent $75,000 on a private investigation to find their daughter but said that investigators' work was stymied by Royal Carribbean, which owns Celebrity Cruises. The family was also disturbed to learn that Carver's cabin attendant, Domingo Monteiro, said he reported the woman missing to his boss for several days but no action was taken. That boss was later fired by the cruise line.
"That was probably one of the worst days of my life, to figure out that they knew Merrian was missing," Carver's father, Kendall Carver, told ABC News in 2006. "If only they had done something during that cruise, when she was reported missing daily, we would have known."
Royal Caribbean later issued a statement saying it appeared Carver had committed suicide on the ship. The statement angered Carver's family.
Kendall Carver said it was possible that his daughter may have committed suicide, but that Royal Caribbean's handling of the case would prevent the family from ever knowing the truth.
Kendall Carver is now the president of the International Cruise Victims Association, a non-profit organization that represents victims of crimes on cruise ships and their families. Read his testimony to Congress on cruise ship safety here.
quicklist: 5category: title: Amy Bradleyurl: media: 15397164text: Amy Bradley, a 23-year-old recent college graduate, vanished in March 1998 while vacationing with her family on a Caribbean cruise. The Bradleys were on the Royal Caribbean Cruise Line's Rhapsody of the Seas, then a year-old luxury ship.
Bradley's parents say they last saw their daughter dancing along with her brother Brad at a Mardi Gras party on the ship before they retired for the night to the family's suite. The ship's computerized door-lock system showed that Brad returned to the suite at 3:35 a.m. March 24, while Amy arrived five minutes later. Brad Bradley said he and his sister sat on the suite's balcony and talked before he went to sleep. He said he last saw Amy sitting in a lounge chair on the balcony.
Ron Bradley said he awoke at 6 a.m. and saw that Amy was not in her room or on the balcony. He searched for her for approximately an hour before he contacted ship security officials. When the ship arrived in Curacao, the Bradley family pleaded with the captain not to lower the gangplank and let its 2,000 passengers off until officials conducted a massive search of the cruise liner.
Crew officials conducted a search of the ship, but found no evidence of foul play and suspected Amy Bradley fell or jumped off the ship. Authorities subsequently searched the ship and the waters but did not find Amy's body. Her whereabouts have been unknown ever since.
Bradley was described as being 5'6" and weighing 120 pounds with short brown hair and green eyes. See her FBI Missing Person page here.
quicklist: 6category: title: Glenn Sheridanurl: media: text: Glen Sheridan, 54, of Williamsburg, Va., was reported missing by his wife in November 2004 after the Carnival Cruise Lines ship Celebration docked in Jacksonville, Fla.
Carnival said at the time that electronic records showed Sheridan had not left the ship after the Jacksonville arrival. A boat and helicopter search of 350 square miles of ocean and St. Johns River turned up nothing, the Associated Press reported.
quicklist: 7category: title: Christopher Caldwellurl: media: 15388915text: Christopher Caldwell, 36, a father of three from Virginia Beach, vanished aboard the Carnival Cruise ship Fascination en route from Miami to Cozumel, Mexico, on July 23, 2004. Caldwell was travelling with his fiancee, who reported him missing after he didn't return to his room one morning. He was last seen hours before on the ship's aft deck.
A Coast Guard search of the South Florida coast failed to find Caldwell.
quicklist: 8category: title: Hue Pham and Hue Tranurl: media: 15388926 text: Hue Pham, 70, and Hue Tran, 65, went missing from the Carnival Cruise ship Destiny in May 2005 as it travelled between Barbados and Aruba. The couple, married 50 years, had been given the cruise by their children as a Mother's Day gift.
Family members travelling with the couple realized they were missing after their shoes were discovered on the ship's deck.
A full search and rescue mission failed to find the couple. Their son, Michael Pham, testified before Congress in 2006 to urge tougher regulation of cruise ship safety practices.
Watch the full story now on "Cruise Ship Confidential," a "20/20" special.