According https://www.thedrive.com/the-war-zone/25728/chinas-new-york-city-sized-earthquake-warning-system-sounds-more-like-way-to-talk-to-subs there is a huge secret ELF-antenna in China called WEM. The exact location is so far unknown, but by the fact, that it looks like an overhead powerline which forms according https://www.newsweek.com/china-giant-antenna-navy-submarine-new-york-city-secret-location-1276332 a cross shape, so it will be able to be mapped by satellite pictures.