What is a spy balloon? China's 'surveillance' device shot down in US

The US on Sunday shot down a high-altitude service balloon belonging to China that was suspected to be spying over strategic sites in the US, as per a statement from Secretary of Defence Lloyd J Austin. A US military aircraft successfully brought down shot the suspected spy balloon off the coast of South Carolina, in an area that didn't risk harming citizens with debris.

While China has angrily voiced its "strong dissatisfaction" at the move and said it may make "necessary responses", here's what we know about spy balloons.

Cheap, quiet, and hard to reach -- these balloons have long been used for reconnaissance purposes, including in conflicts like the American Civil War. The practice became widespread during World War I and was used extensively during the Cold War, when the US launched hundreds of balloons to gather intelligence on the Soviet Union and China.

While their use has declined with the rise of unmanned drones and satellites, many countries still employ spy balloons. The Pentagon is expanding investment in high-altitude inflatables, Politico reported last year. Modern balloons are generally unmanned, but they still generally lack propulsion and are subject to wind currents.