The Integrated Undersea Surveillance System (IUSS) is a system of underwater sonar listening devices that are able to track adversary submarines over very long distances. The system was developed in the 1950s and was very highly classified. Before 1985 it was known as SOSUS. In recent years some information about the system has become public knowledge.
In the early days, long seafloor sonar antennas were laid off the coasts; small facilities dotted up and down the coasts then processed the data from these sonar arrays. A number of Allied nations participated in the program to improve coverage in both the Atlantic and Pacific. The system was able to provide some tracking of Soviet missile submarines in the Atlantic; had the cold war turned hot, this locational data would have been critical in guiding friendly units to attack these Soviet submarines.
Quite a few years ago, the system was revamped so that the data is relayed to centralized processing facilities and the small facilities on the coast were closed.
To fill gaps in coverage, the U.S. Navy built a series of ocean surveillance ships that would tow long sonar arrays hoping to detect and track submarine activity. Five of these still serve in the Western Pacific and the Navy plans to build more as Chinese submarine activity increases. These T-AGOS ships are designed for low speed and stability in high seas with a SWATH design (see photos and attached pdf). Several times over the years, Chinese vessels have interfered with T-AGOS operations. The T-AGOS are unarmed and civilian crewed with a small Navy detachment aboard.
https://en.wikipedia.org/wiki/SOSUS