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noise level is higher or when the signal level is lower than expected. The objective of this article is to show examples of these categories of interference and provide some stories illustrating how they were diagnosed. These underwater acoustic surprises fall into four general categories: (1) External sources: acoustic energy from nonsignal sources in the environment; (2) Environmental propagation effects: changes in transmission loss between the receiver and the source due to sound speed and boundary effects; (3) Internal system noise: sources within a system such as mechanical vibrations, electrical interference, crosstalk, flow noise, and self-noise; or (4) Unexpected reflection: reflection and scattering from water column sources, (e.g., ships, volume inhomogeneities, thin layers, organisms). External Interference Sound is emitted into the water by a wide variety of anthropogenic and natural sources (Bradley and Nichols, 2015), and it is the resulting “external interference” that comes to mind when most people think of underwater noise. Figure 1 shows a few of the many sources of exter- nal interference for acoustic systems in the ocean that result in noisy underwater acoustic data. The background noise level in the ocean is often approximated as a single decibel number based on sea state, weather, and frequency/prevalence of shipping. Although this ambient noise estimate is an approxima- tion of the ambient-noise level, the geographically and temporally variable true ambient noise in the ocean is far more complex. Weird data due to external interfer- ence occur when noise levels exceed expectations and/ or vary significantly with time. Sources of this type of “surprise” external interference in underwater acoustics might include natural sources of sound (e.g., cetaceans, fish calls, snapping shrimp, noise from weather, waves, or geology) and anthropogenic sources of sound (e.g., ships, sonar systems, acoustic modems, pile driving, or airguns). Natural Noise Sources Biological noise sources can cause significant problems with autonomous processing or detection methods because cetacean communications are often in the same frequency band used for underwater communication and navigation systems. Matt Palanze of the Woods Hole Oceanographic Institution (WHOI), Woods Hole, Massachusetts, experi- enced just how challenging it can be to operate acoustical systems in the presence of marine mammal noise when his team was attempting to trigger an acoustic release: “On an OOI cruise, we approached a large surface mooring for recovery. We communicated with the acoustic releases with a deck box located in the ship’s lab. There are three releases on this mooring, two are connected in parallel for redundancy, specifically in the case of a failure. We can release the other with no other intervention being needed. None of the releases would reliably respond to queries and commands; there is a very low historical probability of this occurring. After approximately 20 minutes (which, as you know, is forever in ship time!), one of our colleagues came into the lab and announced, ‘There’s about a thousand dolphins out there!’ We went out on deck, and sure enough, there were dolphins and whales to the hori- zon. We could hear their calls from the deck. We had to stand by for an hour or so until the pods moved on. After that, we released the mooring and recovered as normal. I believe this event went into the cruise report as ‘Operations delayed due to Mammalian Interference’” (personal email, 2022, used with permission). Figure 1. Examples of external interference that may be picked up by a receiver in the ocean. These sources are uncontrolled, and many often occur together to emit sound at varying frequencies into the water that can affect underwater acoustic sensing systems. Summer 2022 • Acoustics Today 35