In 1965, the CIA was installing a nuclear listening device at the summit of Nanda Devi, India’s second-highest peak. The device contained the highly radioactive substance, plutonium.
While the expedition had to be called off due to bad weather, the team left the device there, assuming they'd come back and retrieve it. However, when they returned the following spring, the device was long gone. Till date, people assume the radioactive and powerful device lies in the hands of an unknown person, and so far there has been no official information regarding any of this.