https://en.wikipedia.org/wiki/Blue_Peacock

One proposal suggested that live chickens would be sealed inside the casing, with a supply of food and water. They would remain alive for approximately a week. Their body heat would apparently have been sufficient to keep the mine’s components at a working temperature. This proposal was sufficiently outlandish that it was taken as an April Fool’s Day joke when the Blue Peacock file was declassified on 1 April 2004. Tom O’Leary, head of education and interpretation at the National Archives, replied to the media that, “It does seem like an April Fool but it most certainly is not. The Civil Service does not do jokes.”