Hoover Dam," Thalia said. "It's huge." We stood at the river's edge, looking up at a curve of concrete that loomed between the cliffs. People were walking along the top of the dam. They were so tiny they looked like fleas. The naiads had left with a lot of grumbling—not in words I could understand, but it was obvious they hated this dam blocking up their nice river. Our canoes floated back downstream, swirling in the wake from the dam's discharge vents. "Seven hundred feet tall," I said. "Built in the 1930s." "Five million cubic acres of water," Thalia said. Graver sighed. "Largest construction project in the United States." Zoe stared at us. "How do you know all that?" "Annabeth," I said. "She liked architecture." "She was nuts about monuments," Thalia said. "Spouted facts all the time." Grover sniffled. "So annoying." "I wish she were here," I said.