Iraqi inmates have used bits of plastic, metal and wood to dig two tunnels out of the country's main prison, US officials say.
Camp Bucca holds the majority of Iraq's inmates
One of the tunnels was 200m (600ft) long and went beyond the security fence of Camp Bucca but no-one escaped, US army spokeswoman Maj Flora Lee said.
Troops mounted an extensive search after finding dirt in the toilets and other places in the compound, she said.
Camp Bucca, near the southern town of Umm Qasr, holds over 6,000 inmates.
In other developments:
- Two US troops are killed in a suicide car bomb blast in Baghdad, the US military says
- As many as 131 Iraqi insurgents are detained in an operation near the holy city of Karbala, Iraqi officials say
- A new Iraqi president may be nominated on Tuesday, media reports suggest.
'Nothing of this size'
The attempted jailbreak was extremely elaborate, another US spokesman said.
He said it was believed that the tunnels had been dug over several weeks and prisoners had waited for poor weather and low visibility before trying to make an escape.
A shovel cut from a water container was said to have been used to dig the tunnel.
Maj Lee said the entrance to the tunnel was under a floorboard and was concealed with dirt.
"I'm not aware of any other instances where this has happened," she said.
"There have been a few other attempts at digging a tunnel, but nothing of this size."
US guards said they shot dead four prisoners at the jail in January after a riot following routine search in the cells.