SOS . . . A wireless code-signal summoning assistance in extreme distress, used esp. by ships at sea. The letters are arbitrarily chosen as being easy to transmit and distinguish. The signal was recommended at the Radio Telegraph Conference in 1906 and officially adopted at the Radio Telegraph Convention in 1908. (See G. G. Blake Hist. Radio Telegr., 1926, 111-12).
The Oxford English Dictionary

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Billy Reston glides into frame, paying no attention to the equipment which Navidson over the last few weeks has been setting up in the living room, including though not limited to, three monitors, two 3/4" decks, a VHS machine, a Quadra Mac, two Zip drives, an Epson color printer, an old PC, at least six radio transmitters and receivers, heavy spools of electrical cord, video cable, one 16mm Arriflex, one 16mm Bolex, a Minolta Super 8, as well as additional flashlights, flares, rope, fishing line (anything from braided Dacron to 40 lb multi-strand steel), boxes of extra batteries, assorted tools, compasses twitching to odd polarities in the house, and a broken megaphone, not to mention surrounding shelves

already loaded with sample jars, graphs, books, and even an old microscope.

Instead Reston concentrates all his energies on the radios, monitoring Holloway as he makes his way through the Great Hall. Exploration #4 is underway and will mark the team's second attempt to reach the bottom of the staircase.

"We hear you fine, Billy" Holloway replies in a wash of white noise.

Reston tries to improve the signal. This time Holloway's voice comes in a little clearer.

"We're continuing down. Will try you again in fifteen minutes. Over and out."

The obvious choice would have been to structure the segment around Holloway's journey but clearly nothing about Navidson is obvious. He keeps his camera trained on Billy who serves now as the expedition's base commander. In grainy 7298 (probably pushed one T-stop), Navidson captures this crippled man expertly maneuvering his wheelchair from radio to tape recorder to computer, his attention never wavering from the team's progress.