Lost in the Tunnel of Time (Clubhouse Mysteries, Bk. 2)

In Lost in the Tunnel of Time, the boys discover their hometown was a stop on the Underground Railroad and are excited to explore the part of the tunnel that is right under their school. When a trapdoor slams behind them, locking them in the tunnel, there's only one thing they can do - plunge deeper and deeper into the darkness. Where will the tunnel lead them? And will the old, crumbling walls hold until they find their way to the end?