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Finally, the digging has begun.

Four freezing days and nights after a huge boulder crushed a highway tunnel in northern Japan and trapped 20 people, workers succeeded Wednesday in blasting the rock into small enough pieces to begin digging their way in.A cluster of backhoes, their bright orange and yellow hues standing out sharply against a towering hillside of gray debris, began shoveling scoops of the broken-up rock from the top and side of the tunnel and dropping it into the sea.

Authorities said it could still take days to dig into the shattered tunnel, outside a remote cliffside village about 550 miles north of Tokyo.

Even though hopes of finding survivors have faded, the rescue drama has captured national attention since Saturday, when a slab of mountain the size of a 20-story building peeled free and crushed the tunnel. A bus with 19 aboard and a car with one occupant were trapped.

Wednesday's blast, the fourth attempt to topple or blow up the rock, reduced the boulder to a huge pile of loose debris and sent two big chunks rolling into the ocean. Officials did not rule out the possibility of another blast if the backhoes uncover slabs too big to be removed.

Meanwhile, questions were growing over safety standards for similar tunnels. Japan's Kyodo News reported Wednesday that 18 months ago, not far from the accident site, a rock twice as big plunged down from a mountainside, though it did not strike a roadway.

The report suggested authorities failed to heed warning signs of instability in the area. Officials could not immediately be reached for comment.

On Tuesday, Prime Minister Ryutaro Hashimoto ordered nationwide checks of tunnels located near cliffs or other potentially unstable formations.

Officials in Hokkaido said they conduct daily tunnel patrols and had checked the tunnel the day before it was crushed. Nothing unusual was noted at the time, an official said, but added that the checks were not exhaustive.

"We take a brief look at the ceiling and entrance of the tunnel, but it can't really be called an inspection," said Yasuhiro Kasai, a construction official with the Hokkaido Development Bureau.

The last full-scale inspection of the tunnel was in September 1991, said Naotoshi Baba, director of the road disaster prevention division at the Ministry of Construction. It found nothing wrong.

It wasn't clear why the slab of rock fell.