The Woman in the Tunnel

S1, E16: When the remains of a young documentary filmmaker are found in a ventilation shaft in an underground Washington, DC, tunnel, Brennan and Booth must venture into the maze of tunnels beneath the city and are surprised to discover a world of homeless shelters underground. Clues lead them to suspect that a man living underground was involved with the murder, but instead he gives them valuable information about the purpose of the victim's underground project.