noun
drift bottle
A bottle released into the sea to study ocean currents, often containing a message or tracking information.
海流瓶を海に流して、海流の動きを調べる。
They release drift bottles into the sea to study the movement of ocean currents.
Compound of 海流 (かいりゅう, 'ocean current') and 瓶 (びん, 'bottle').