Dec. 30, 2011 -- intro: Few things pique the human curiosity, suspicion and fear like the realization that someone has disappeared, particularly if the person is a small child.

In 2011, several cases captured the nation's attention, including the disappearances of infant baby Lisa Irwin in Missouri and toddler Sky Metalwala in Washington, along with adults like Robyn Gardner, who vanished in Aruba.

Here are 11 of the most intriguing missing persons cases of 2011:

quicklist: 1 category: Baby Lisa Irwin title: Lisa Irwin, 1 url: media: 14673723 text: Police are baffled over the case of Lisa Irwin, an infant who disappeared from her crib in the middle of the night in Kansas City, Mo., and would have turned 1 on Nov. 11.

Baby Lisa was last seen asleep in her crib when her mother, Deborah Bradley, said she checked on her at 10:30 p.m. on Oct. 3. The infant's father, Jeremy Irwin, discovered Lisa was missing when he arrived home at 4 a.m. the following morning after working the overnight shift. The parents immediately called the police.

Police have been suspicious of the parents' story since their initial questioning. Bradley said that police accused her of having done something to her child, and also accused her of failing a polygraph test.

The relationship between police and the parents has soured over the course of the investigation. Authorities brought in cadaver dogs as part of a 17-hour search of the home, where police say the dogs "indicated a positive 'hit' for the scent of a deceased human on Oct. 21," but attorneys for the family have questioned their motivations.

The Kansas City Police Department requests that anyone with information regarding the baby's whereabouts to please call their tips hotline: (816) 474-8477.

quicklist: 2 category: Missing Maine Toddler title: Ayla Reynolds, 20 Months url: media:15205938 text:In a case that has become all too familiar this year, 20-month-old Ayla Reynolds of Waterville, Maine, vanished from her crib in the night.

According to police, the toddler's father, Justin DiPietro, said he put the child to bed the night of Dec. 16 in his home, and awoke the next day to find her missing. Police have said several other adults were in the home that night, including one non-relative. Ayla was living with her father while the girl's mother, Trista Reynolds, 23, was in rehab for substance abuse.

Trista Reynolds' family said the 20-month-old had been taken from their custody by the Department of Health and Human Services and placed with her father. Trista Reynolds said the baby, who broke her arm at her father's house two weeks ago, had begun showing mysterious bruising on her leg. While DiPietro did not comment on the girl's alleged injuries, he disputed the Reynolds family's account of Ayla's custody. The FBI is now involved in the case.

Anyone with information is asked to call the Waterville Police Department or or the National Center for Missing & Exploited Children, 1-800-843-5678 (1-800-THE-LOST).

quicklist: 3 category: Maryland Woman Missing in Aruba title: Robyn Gardner, 35 url: media: 14283094 text: Maryland native Robyn Gardner was last seen on Aug. 2, at the Rum Reef Bar & Grill in the Baby Beach area of Aruba.

Her travel companion, Gary Giordano, 50, claimed Gardner was swept out to sea when the two went snorkeling. Suspicions were raised by the $1 million travel insurance policy on Gardner that named Giordano as the beneficiary.

After spending four months in an Aruban jail, Giordano was released and returned to the United States. No charges have been brought against Giordano, and Aruban prosecutors have not found a body.

In a Dec. 4 interview with "Good Morning America," Giordano, who remains the lone suspect in Gardner's disappearance, said he believed human traffickers may be behind it, but he couldn't explain how they might have made off with the woman.

If you have any information regardling Gardner's disappearance, submit a tip through the FBI's Tips and Public Leads website: https://tips.fbi.gov/

quicklist: 4 category: Missing IU Student title: Lauren Spierer, 20 url: media: 13809703 text: Lauren Spierer, 20, was last seen on June 3 leaving a sports bar at 4:30 a.m. in Bloomington, Ind., where she had just finished her sophomore year at Indiana University.

The investigation into the disappearance of the petite fashion major from Greenburgh, N.Y., has taken many turns, yielding thousands of tips and at least 10 persons of interest.

In the weeks immediately after she vanished, some of Spierer's friends came under scrutiny for not coming forward voluntarily to provide information to the police. Her parents, Robert and Charlene Spierer, believe their daughter was abducted and are offering a reward of $100,000 for information that leads to finding her.

Months later, little remains known about what might have happened to Spierer.

The Bloomington Police Department is asking anyone with information about her whereabouts to call 812-339-4477.

quicklist: 5 category: Missing Nursing Student title: Holly Bobo, 20 url: media: 14677055 text: Nursing student Holly Bobo, 20, has been missing since April 13 when a man in camouflage marched her into the woods near her home in rural Decatur County, about three hours from Nashville, Tenn.

Her brother Clint, 25, saw her go into the woods, but mistakenly believed the man with her was her boyfriend. Police have called off additional searches and have no suspects in her disappearance.

Country singer Whitney Duncan, who is Bobo's cousin, expressed anger toward investigators on Twitter, tweeting that she was "frustrated as you can imagine any family would be."

Bobo has blond hair and blue eyes. She was last seen wearing a pink shirt and light-blue jeans.

The Tennessee Bureau of Investigation is asking anyone who might have any information about this case to please call the TBI Tips Hotline: (800) 824-3463.

quicklist: 6 category: "Peoples' Court" Mom title: Michelle Parker, 33 url: media: 14992878 text: Michelle Parker had a moment in the spotlight when she appeared on the "Peoples' Court" TV show on Nov. 17 along with her ex-fiance Dale Smith in a dispute over a $5,000 engagement ring.

Hours later she dropped off their twins at Smith's Orlando, Fla., house and she hasn't been seen since.

Parker's cell phone was discovered in early December at the bottom of a lake near Smith's house, prompting fears by Parker's family that foul play was involved in her disappearance.

Police have said Smith is their prime suspect, but no charges have been filed and a judge said their is not sufficient evidence to remove the twins from his custody.

quicklist: 7 category: Missing Wash. State Toddler title: Sky Metalwala, 2 url: media: 14896824 text: There is still no sign of 2-year-old Sky Metalwala since he disappeared on Nov. 6, and police say his mother's account of what happened on the day her son disappeared is suspicious.

The FBI joined the search for the toddler after his mother, Julia Biryukova of Redmond, Wash., said she left him alone for an hour in a disabled car in Bellevue, Wash.

Biryukova told police she was driving with her two children when she ran out of gas. She and her 4-year-old, Maile Metalwala, trekked about a mile to a nearby gas station, leaving Sky in the car, she said. According to police, Biryukova did walk to the gas station that morning, but she never bought gas there, and that her story is "strikingly similar in nature" to a recent "Law & Order" episode.

Sky's father Solomon Metalwala, who is separated from his wife, told ABC News that he believes she is responsible for their son's disappearance. The couple has a troubled history that includes a citation for leaving Sky alone in a parked car at a Target parking lot in 2009, and angry accusations hurled at each other during their divorce proceedings.

Anyone with information regarding Sky should contact the Bellevue, Wash., Police Department, 1-425-577-5656, or the National Center for Missing & Exploited Children, 1-800-843-5678 (1-800-THE-LOST).

quicklist: 8 category: Missing W.Va. Girl title: Aaliyah Lunsford, 3 url: media: 14626657 text: The FBI announced in October that the search for missing 3-year-old Aaliyah Lunsford, who disappeared from her West Virginia home Sept. 24, would be treated as a criminal investigation.

Aaliyah was last seen when her mother checked on her around 6:30 a.m., but was gone by 9:30 a.m., according to ABC affiliate WDTV.

The Lewis County Sheriff's Department is asking anyone with information to please call (304) 269-8251.

quicklist: 9 category: Missing Arizona Child title: Jahessye Shockley, 5 url: media: 14733104 text: The mother of a missing 5-year-old Phoenix girl was arrested in connection to her disappearance.

Jerice Hunter, the mother of Jahessye Shockley, was arrested on Nov. 21 on charges of child abuse "directly related to her daughter Jahessye," according to the Glendale, Ariz., Police Department.

The police and FBI served a second search warrant on Hunter's apartment complex, where Jahessye disappeared Oct. 11, and arrested Hunter there.

According to Officer Tracey Breeden, a Glendale Police Department spokesperson, Jerice Hunter called 911 after she returned from running a quick errand and discovered the girl missing around 5 p.m. on Oct. 11. Jahessye had been at home with her three siblings, ages 6, 9 and 12.

Breeden added that the mother said when she got home, the front door was open, but Jahessye wasn't inside and the siblings did not know where she was.

The Glendale Police Department is asking anyone with information regarding the little girl's whereabouts to please call its tip line at 623-930-HELP (4357).

quicklist: 10 category: Arkansas PoliceDispatcher Missing title: Dawna Natzke, 46 url: media: 15258981 text: Dawna Natzke, a police emergency dispatcher and mother of three, vanished days before Christmas.

Natzke's son reported her missing Dec. 23 and her charred car was found on Dec. 24 in a remote part of Oauchita National Forest, but police did not begin searching the area until after Christmas.

Natzky, who lived in Hot Springs, Ark., worked at the Hot Springs Village police department as an emergency dispatcher for the past six years.

"She's family, and we're doing everything we can do to find her," Lt. Ricky Middleton of the Hot Springs Village police told ABC News.

Natzke's live-in boyfriend reportedly told police that she was watching television alone when he went to bed around 11 p.m. on Dec. 21. That was the last time anyone saw her.

Although there has been an intensive search for Natzky this week, with dogs and police on horseback, there is no sign of her.

Editor's note: Natzke's body was found over the New Year's weekend in a Arkansas forest. Her death was ruled a homicide.

quicklist: 11 category: Possible Craigslist Link title: Amy Ahonen, 38 url: media: 14097188 text: Amy Ahonen was last seen on July 8, the day before her 38th birthday.

Her car was found on the side of Highway 6 in Jefferson County, Colo., with her belongings in it, including the keys in the console, her purse under the seat and her hiking backpack in the vehicle. The car was left unlocked.

Andrea Ahonen says her sister had an frightening experience with a man several months before she disappeared when she used Craigslist to advertise for a roommate, and she has told police about it.

Hoping to save some money, Amy Ahonen posted an ad on Craigslist looking for a roommate. In March, a man who responded came to look at the place and "tried to kiss her," according to her sister.

"She got very freaked out about it and took the ad down," Ahonen said.

For more information on people missing in America or to offer tips contact the National Center for Missing & Exploited Children or the Federal Bureau of Investigations.