In 1963, the U.S. government established a research base on Lake Vostok in Antarctica. The team of engineers and scientists were tasked with drilling into the subglacial lake and researching the life within. A second, clandestine reason was its potential use for the military.
In 1966, the team began drilling, and by April of 1966, they reported breaking through the ice. The base fell silent. A research operation went underway, but just before its launch in late May 1966, the base sent out one last transmission.
The contents of the tape were mostly purged after its reception, but the surviving audio gives a clear warning: "Stay away from Lake Vostok."
Whatever the contents of the rest of that tape, the U.S. government did something which it had never done before, and which it hasn't done since.