On Monday, workers started working the pumps again at 9 a.m. Balkowitsch said he didn’t want to use electrical jacks because he wouldn’t be able to control the pressure closely enough. He said the slab had four or five fractures running through it, and he was afraid that, if he pushed too quickly, the top half would go over and leave the base intact.

In addition to the jacks, on Monday he rigged up Plan B — a dump truck full of gravel attached to a cable, which was attached to a steel plate bolted to the back side of the slab. If the jacks couldn’t do the trick, he planned to use the truck — weighing 50,000 pounds loaded — to pull the slab off the Rims.

He was on the verge of using Plan B when, about 17 minutes after his workers started, they reported that the pressure on the jack was dropping almost to nothing. By then they had already moved the slab another 2 feet.

Balkowitsch said a worker told him by radio, “We’re going into failure. We’ve got zero in the pump.”

Within a minute, amid cheers from the workers up top, the slab started coming forward. The top half hit the slope below the Rims with a great rolling whump and a huge cloud of dust. Then the bottom half rolled over the top, breaking into smaller fragments.