Frank turned to Hazel. “Can you do that thing again?”
She punched him in the chest. “You said you wouldn’t tell anybody!”
Immediately Frank felt terrible. He’d gotten so caught up in the idea...
Hazel muttered under her breath. “Never mind. It’s fine. Percy, he’s talking about the trenches. The Field of Mars is riddled with tunnels from over the years. Some are collapsed, or buried deep, but a lot of them are still passable. I’m pretty good at finding them and using them. I can even collapse them if I have to.”

The Son of Neptune, Chapter X