The sinkhole appeared in the middle of the southern city of Fukuoka last Tuesday (Nov 8), swallowing part of a five-lane street near the main railway station.
The gaping hole, around 30 metres wide, exposed support columns of nearby buildings at a traffic intersection, prompting fears of further collapses.
TV footage showed two separate holes steadily expanding as they sucked in asphalt and soil in front of the city’s Hakata Station.
“It was a bit dark outside, and my first impression was, ‘Is the road really falling?’” a young man who saw the sinkhole develop told public broadcaster NHK.
“When I saw it, two holes were already there, and they continued to grow bigger.
“I got scared the most when a traffic light fell at an intersection close to where I was standing. I felt, ‘I have to get out of here,’” he told NHK.
The hole continued to expand and by afternoon was around 15 metres deep, and filling with water from broken pipes.
Police, who had cordoned off the area and evacuated buildings, said there had been no reports of any injuries.
Fukuoka’s mayor and transport bureau said the ongoing extension to a nearby subway line might have triggered the sinkhole.
Construction had to stop early morning when workers saw ground water washing into the site, shortly before the road began to cave in, according to local media.
Fukuoka Mayor Soichiro Takashima told a press conference the city was scrambling to prevent further damage.
“We believe the subway construction had some impact,” he said.
“A thorough investigation will be carried out,” he added.
Fukuoka is the biggest city on the southernmost main Japanese island of Kyushu.
Its railway station is a major intersection, including for the superfast bullet trains that criss-cross the country.
Japanese workers toiled continuously for a week, dumping huge amounts of wet cement and sand into the gaping hole and fixing electricity, gas and water lines that had stopped following the accident.
About one-third of the huge hole had been filled in by Wednesday afternoon.
“The hole stopped expanding and stabilised by yesterday afternoon. About 35 per cent of the 7,000 cubic-metre hole will be filled this afternoon,” a city official said.
“Crews are working around the clock.”
The street reopened at 5am Tuesday, Takashima said in a statement, exactly a week after the ground began sinking.
“We’re very sorry for causing great trouble,” Takashima said, noting that a ban on access to surrounding buildings was lifted.
Miraculously, no one was hurt in the accident, in the biggest city on the southernmost main island of Kyushu.
Many on social media expressed amazement at the quick recovery.
“I’m surprised the road reopened in a week!” one Twitter user said.
“Impressive. That was fast,” said another.