Envelope reads "Hetch Hetchy Calaveras tunnel explosion 6/9/30 Sam R Hensley." Photograph shows a man posing with arms crossed. He wears a suit or sports coat and a hat. He appears to be standing in front of a car or other vehicle. On June 8, 1930 seven men were killed at the Calaveras Tunnel construction site when a pocket of gas was ignited by sparks from an electric tram. The blast was so powerful that men on the surface above the unfinished tunnel "believed an earthquake had occurred." Damage to equipment was suffered as far away as one mile from the blast site, and near the center of the explosion a 100 pound transformer was thrown 500 feet through the air. Initial rescue efforts were foiled by toxic gasses which still filled the bore. The Calaveras Tunnel was part of the Hetch Hetchy dam construction project. A 5,000 foot bore through the hills near Livermore in Alameda County would be used to carry water as it traveled from the resevoir in the Sierra Nevada mountains to San Francisco. Hetch Hetchy, valley in the northwest part of Yosemite national Park, bearing an indian name for its food crop of grass seed or acorns, watered by the Tuolumne River. To supplement the Spring Valley water supply for San Francisco, City Engineer C. E. Grunsky persuaded Mayor Phelan to request the federal government's permission to dam the river and flood the valley (1905). The proposal was opposed by conservationists led by john Muir as well as by Modesto farmers who feared the loss of their irrigation water. President Roosevelt endorsed the resevoir (1906), but President Taft's Secretary of the Interior Ballinger withdrew sapproval (1910). Finally President Wilson and his Secretary of Interior Franklin Lane gave permission to proceed, and the water system, designed by M.M. O'shaughnessy, was in full operation by 1934. The dam, completed in 1923, was heightened in 1938. desite the Raker Act (1912) of Congress prohibiting private firms from distributin its power, San Francisco voters granted that right to the Pacific Gas and Electric Co. (1925). -- From James D. Hart, A Companion to California, University of California Press 1987, p214-215. Information about the explosion taken from articles in the Oakland Tribune on June 9, 1930. This photograph was printed in the Oakland Tribune on June 9, 1930 (p. 3) with the caption "Sam R. Hensley, who was nearly overcome when he went into the tunnel without a gas mask intent on rescuing his army 'buddy,' Carl C. Cook.