Cementing fills and seals the annulus between the well casing and the drilled hole.

FORBES: The Fracking Solution Is A Good Cement Job

As part of the kill process, the workers drilled into the annulus, or the outer ring of the well.

CNN: STORY HIGHLIGHTS

Its tubercles and soft tissues were inflamed, and the annulus fibrosus of its disks was the color of bad teeth.

NEWYORKER: Backbone