Envelope reads "Hetch Hetchy Calaveras tunnel explosion June 1930." Photograph shows the entrance to a tunnel with approximately nine men grouped around it. The men are dressed in a variety of clothing including overcoats, suits, hats, and overalls. Above the entrance to the tunnel are signs warning against smoking or carrying matches into the tunnel. To the left of the frame a pipe can be seen protruding vertically from the ground and to the right of the frame some sort of controll box can be seen next to a stone wall. For more information about the Calaveras Tunnel and the Hetch Hetchy project see h99.1.470.