Giant steel pipe's mysterious overnight growth spurt baffles a Japanese city

Japanese authorities were investigating Friday how a large steel cylinder suddenly emerged from the ground during sewer construction work and rose to the height of a four-storey building in the city of Osaka.

A city official told AFP they had received a report early Wednesday that the object, a steel casing used for soil retention, was "jutting out of the ground" near highways in Osaka.

"It was not there the previous day," he said.

The incident comes after a massive sinkhole near Tokyo -- that swallowed a truck and its driver -- appeared last year, highlighting the risk posed by Japan's ageing infrastructure.

The cylinder -- with a diameter of around 3.5 metres -- rose 13 metres into the air, but had sunk to a height of around 1.6 metres by Friday morning after workers rushed to pour water inside to help submerge it.

Two roads in the vicinity -- both of which lead to city centre districts -- have been closed to traffic since Wednesday, causing congestion, the city official told AFP.

But one reopened Friday afternoon and the other will be opened "soon", with city officials considering cutting off the remaining exposed portion of the cylinder, he said.

"We are still investigating the cause of the trouble," he added.

The incident comes after Osaka announced last month it had received an unusual donation of 21 kilogrammes of gold to pay for the maintenance of its ageing water system.

The donation worth $3.6 million was made in November by a person who had already given $3,300 in cash for the municipal waterworks, Osaka Mayor Hideyuki Yokoyama told a press conference.

Work to replace water pipes in Osaka, a city of 2.8 million residents, have hit a snag as the actual cost exceeded the planned budget, according to local media.

Prime Minister Sanae Takaichi has said her "responsible and proactive" fiscal spending would include investment in infrastructure.

Agence France-Presse