The Hidden Tunnel (Mystery House #4)

Ellen, Sue, and Tanya travel to Portland, Oregon to help a local billionaire look for his missing brother, because the authorities have come up empty. In exchange for their help, the billionaire gives them a haunted mansion, where his brother was last seen and where they learn about the old Shanghai tunnels connecting Chinatown to the city. As they dig deeper into a local legend, they discover an obscure, unknown tunnel connecting to their renovation. Although hauntings in the tunnel might explain the strange happenings, they soon fear that the cause is more sinister and more powerful than ghosts.