“There’s an opening just ahead,” Hazel announced. “We’ll come up ten feet from the east wall.”
“How can you tell?” Percy asked.
“I don’t know,” she said. “But I’m sure.”
“Could we tunnel straight under the wall?” Frank wondered.
“No,” Hazel said. “The engineers were smart. They built the walls on old foundations that go down to bedrock. And don’t ask how I know. I just do.”

The Son of Neptune, Chapter XI