Hey there, I have been doing research with the Carlsbad Caverns National Park on the history of Caverns. So far, the earliest account of an Underground Lunch Room dates to 5/16/1928. This is the date recorded for when the Cavern Supply Company began serving lunches at the Lunch Room's original location at the Big Room Junction. This was recorded in the Superintendent's Monthly Report for May 1928. Hope this helps!