I am reading an article on Science about collecting sound of underwater species, which is in the Q&A form. Part of this article says:
Q: Do your hydrophones ever … float away?
A: Oh yeah. In a freshwater bay, I had one I’m pretty convinced someone dragged away. We don’t mark them—otherwise curious people pick them up—but I had a ground line running along [this] hydrophone so I could find it later. I think someone [accidentally] put their anchor down and dragged it off somewhere.
I don't understand what the interviewee mean by "ground line" here. I cannot imagine what "a ground line running along [this] hydrophone" describes.