Indonesian engineers have lowered the first series of concrete balls into a mud volcano to try to plug an eruption which has destroyed thousands of homes.
Engineers are unsure how long the plugging process will take
At least four of the giant, chain-linked spheres were dropped into the crater, but technical problems halted work at the East Java site.
Workers plan to drop 1,500 balls, each weighing up to 250kgs (500lbs).
Some scientists say the mud geyser was likely triggered by gas drilling, but the gas company blames an earthquake.
The geyser began spouting the noxious muck nine months ago at a gas drilling site in East Java.
It has since buried factories and thousands of homes, and displaced an estimated 13,000 people.
Experts warn the torrent could continue for months, if not years, to come.
The Indonesian government has been working to halt the mud with a network of dams and by channelling some of it into the sea, but with little success so far.
A team of geologists and engineers hope the plan, believed to have never been tried before, will reduce the amount of mud flowing from the site by up to 70%.
The mud leak has submerged several villages
Each 1.5m-long metal chain has four concrete balls suspended from it; two with a 40cm (16 inch) diameter and two with a 20cm diameter.
The team had planned to begin slowly, dropping five to 10 chains on the first day, then increasing the number until they insert up to 50 chains per day.
But the process was halted on the first day of the operation when a steel cable hoisting the balls broke, officials quoted by the Associated Press said
The disaster began on 29 May 2006 in the Porong sub district of Sidoarjo in Eastern Java, close to Indonesia's second city of Surabaya.
Some independent scientists believe it was triggered by the drilling work of gas prospectors PT Lapindo Brantas.
Other research supports the company's assertion that it was a natural disaster resulting from increased seismic activity following a major earthquake two days before the mud began flowing.