Frank’s vision went red with anger.
“Let’s shake things up.” He reached in his quiver and pulled out an arrow heavier than the rest. The iron tip was shaped like the nose cone of a rocket. An ultra thin gold rope trailed from the fletching. Shooting it accurately up the wall would take more force and skill than most archers could manage, but Frank had strong arms and good aim.
Maybe Apollo is watching, he thought hopefully.
“What does that do?” Percy asked. “Grappling hook?”
“It’s called a hydra arrow,” Frank said. “Can you knock out the water cannons?”

The Son of Neptune, Chapter XI