Spy balloons: What are they?

A suspected "Chinese spy balloon” was seen flying across the US. Canadian officials have also confirmed the sighting and monitoring of a high-altitude surveillance balloon, later on Thursday (2 February). The balloon spotted first in the city of Billings in Montana is currently flying at an altitude “well above commercial air traffic” and is not “a military or physical threat to people”, according to the Pentagon Press Secretary Brig. Gen. Pat Ryder. Chinese officials have claimed they are "verifying the facts".