Weather balloons have been drifting toward Taiwan for at least seven weeks in what defense experts say is China’s latest tactic to keep up pressure on the self-ruled island, which it has vowed to someday take over.

An animated map compiled by Newsweek traces the paths of 58 balloons that have been detected crossing the Taiwan Strait‘s median line since December 7, the date Taiwanese authorities began reporting them. As of early Friday, around half of the balloons had violated Taiwan’s territorial air space, including 16 that flew over the island itself, according to the Defense Ministry.