Hoover Dam," Thalia said. "It's huge."<br />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.<br />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.<br />"Seven hundred feet tall," I said. "Built in the 1930s."<br />"Five million cubic acres of water," Thalia said.<br />Graver sighed. "Largest construction project in the United States."<br />Zoe stared at us. "How do you know all that?"<br />"Annabeth," I said. "She liked architecture."<br />"She was nuts about monuments," Thalia said.<br />"Spouted facts all the time." Grover sniffled. "So annoying."<br />"I wish she were here," I said.