China Daily | Chinese underwater acoustic communication machine achieves long-distance transmission, 2021

The acoustic signal at a lower frequency can be transmitted farther underwater, said Qu Fengzhong, a professor with Zhejiang University. But the bandwidth available for communication at low frequencies is very limited. Thus it is difficult for underwater acoustic communication to meet the requirements of high speeds and long distances.

SCMP | Chinese scientists develop long-distance underwater communication in South China Sea, 17 Sep 2022

Chinese researchers say they have developed and tested in the depths of the South China Sea underwater communication technology that would allow submarines and drones to maintain contact over more than 30,000 sq km (11,600 square miles) - about the area of Belgium. ...

historicity I've got a fun story about low-freq communication in NYC c. 2001 that stopped my free-range 2 y.o. Z-informant and a buncha adults in their tracks at the intersection of Houston and B'way ...