The upstate New York prison break of two inmates who squeezed through underground pipes last weekend was among a handful of recent daring escapes by US convicts whom experts describe as ingenious and willing to risk it all to flee a life behind bars.

Escape schemes as cunning as slipping out of prison in a mail sack or using a can opener to cut through the ceiling of a jail house kitchen have been employed in the past decade.

Escapees have got "imagination, they're creative, they're intelligent and they're willing to risk their lives," said Charles Ewing, a law professor at University at Buffalo who specializes in forensic psychology and violent behaviour.

Jeff Hall, a history professor at Queensborough Community College who has studied prisons blamed cutbacks in education programs aimed at rehabilitating prisoners, such as those offering high school and college degrees, for creating a void in which escapes serve as one of the only mental challenges.

"Opportunities for wild imaginings becoming reality are greater now that prisoners' minds are effectively being wasted and their potential abilities and gifts are being squandered," Hall said.

Authorities for a seventh day on Friday are searching for convicted killers Richard Matt, 48, and David Sweat, 34, who were discovered missing from Clinton Correctional Facility in Dannemora, New York, about 5:30 a.m. on June 6. They had busted through metal walls, crawled through pipes and popped out a manhole on the street. On their way out, they left a taunting note, "Have a nice day!"

Similarly audacious was a December 2007 escape by two prisoners from a New Jersey lockup who left behind a note, "Happy Holidays!" with a drawing of a character making an obscene hand gesture. Inmates Jose Espinosa and Otis Blunt used sharpened wire and the handle from a steam pipe to chip their way out of cinder block cells at Union County Jail, jumping from a roof and scrambling over a razor-wire fence. The convicts, who had covered their handiwork with magazine photos of bikini clad women and stuffed their beds with decoys, were captured the following month.

In a brazen escape from an Ohio state prison in September 2014, inmate TJ Lane scaled a 13-foot ladder he spent months fashioning out of old cabinets and other materials that he'd squirreled away in a crawl space. Lane, who was serving a life sentence for killing three teenagers in a high school shooting, was caught the morning after his breakout in wooded area near the prison.

Life on the lam lasted a whole five months for Ralph "Bucky" Phillips, who escaped western New York's Erie County jail in April 2006 by using a can opener to cut a hole in the kitchen ceiling and fleeing through the roof. After a massive manhunt, during which Phillips killed a state trooper, he surrendered in Pennsylvania.

Perhaps one of the boldest breakouts of the last decade was carried out by Richard Lee McNair, whose April 2006 escape from a federal penitentiary in Pollock, Louisiana, on a pallet of mailbags was recounted in Canadian journalist Byron Christopher's book "The Man Who Mailed Himself Out of Jail." McNair, who had been serving time for a murder in North Dakota and whose prison job was to repair old mailbags for the U.S. Postal Service, was captured 18 months later in Canada.

Whatever the cause, there's no question that the monotony of prison life makes the dream of escape a tantalizing mental exercise for inmates, said Sean Kelley, a senior vice president at Eastern State Penitentiary Historic Site in Philadelphia.

"An escape plan is the ultimate challenge - intellectual and physical," Kelley said. "Ironically, perhaps, it helps pass the time." - Reuters