Indonesian engineers on Monday dropped five clusters of chained concrete balls in the mud volcano, which has inundated entire villages and displaced over 10,000 people since erupting near the Banjar Panji-1 exploration well last May.

Numerous other efforts to cap the flow have failed.

AAP yesterday quoted the operation’s spokesman Rudi Novrianto as saying the concrete balls slid 1km into the crater, roughly twice the depth anticipated, so many more than planned may be required to halt the mudflow.

“Based on our monitoring of Monday’s operations, we may later decide to add to the number of ball chains, but the decision will only be made once the initial target of 374 chains have been dropped into the mud hole,” Novrianto said.

Each chain comprises four concrete spheres weighing 400-500kg.

The Koran Tempo newspaper quoted Basuki Hadimuljono, head of the team trying to plug the crater, as saying the number of chains required may grow from the initial estimate of 374 to 1000.

Some experts have expressed doubt about the method, saying pressure from below could push the mud to come out from somewhere else.

Scientists are also unsure how long the volcano will continue to spew mud if left unchecked, but some have said it could continue for decades.

Australian company Santos holds a non-operating 18% stake in the Banjar Panji exploration well venture, in partnership with Indonesian operator Lapindo Brantas and PT Medco Energi.