Where do you find literal references in the book 'Holes' , that explain the title?

Out on the lake, rattlesnakes and scorpions find shade under rocks and in the holes dug by the campers.

The lake was so full of holes and mounds that it reminded Stanley of pictures he'd seen of the moon.

Everyone else's hole was a lot deeper than his. He couldn't actually see their holes but could tell by the size of their dirt piles.

The surface of the table reminded Stanley of the surface of the lake. It was full of bumps and holes because so many people had carved their initials into the felt.

After a while he'd lost track of the day of the week, and how many holes he'd dug. It all seemed like one big hole, and it would take a year and a half to dig it.