One of history’s most notable clandestine acts is the “Great Escape” from the Nazi prison camp, Stalug Luft III. Six hundred airmen imprisoned within the camp collaborated to build three massive tunnels, each carving an escape route 30 feet underground. They discreetly started the tunnels in a bathroom drain, underneath a stove, and in a dark corner of a hallway. They grew them to the point where they had installed air pumps, electrical lighting, and rail cars, which were needed to remove an estimated 200 tons of soil. Despite suspicions that something major was going on, the Nazis never caught the prisoners constructing the tunnels.
Kept or done in secret, often in order to conceal an illicit or improper purpose.
From Latin adverb form of celare, meaning “to hide”, and possibly intestinus, meaning “internal”. Celare is also the root of cell, ceiling, conceal, and occult.
undercover, cloak-and-dagger, surreptitious
Wikipedia: Stalug Luft III