We present a record of ambient sound obtained using a unique deep-ocean instrument package and mooring that was successfully deployed in 2015 at Challenger Deep in the Mariana Trench. The 45 m long mooring contained a hydrophone and an RBR™ pressure-temperature sensor. The hydrophone recorded continuously for 24 days at a 32 kHz sample rate. The pressure logger recorded a maximum pressure of 11,161.4 decibars, corresponding to a depth of 10,829.7 m, where actual anchor depth was 10,854.7 m. Observed sound sources included earthquake acoustic signals (T phases), baleen and odontocete cetacean vocalizations, ship propeller sounds, airguns, active sonar, and the passing of a Category 4 typhoon. Overall, Challenger Deep sound levels in the ship traffic band (20–100 Hz) can be as high as noise levels caused by moderate shipping, which is likely due to persistent commercial and military ship traffic in the region. Challenger Deep sound levels due to sea surface wind/waves (500 Hz to 1 kHz band) are as high as sea state 2, but can also be very low, equivalent to sea state 0. To our knowledge, this is the first long-term (multiday to week) broadband sound record, and only the fifth in situ measurement of depth, ever made at Challenger Deep. Our study indicates that Challenger Deep, the ultimate hadal (>6,000 m) environment, can be relatively quiet but is not as acoustically isolated as previously thought, and weather-related surface processes can influence the soundscape in the deepest parts of the ocean.

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Figure S1 (197 KB pdf)
(left) Schematic of a National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory (NOAA/PMEL) deep-ocean hydrophone and pressure sensor mooring. The mooring is 45 m in length, composed of (1) top mast with satellite beacon, (2) nine glass floats encased in plastic “hardhats,” (3) hydrophone and pressure sensor in double-yoke frame, and (4) a dual acoustic release. (above) Schematic of the PMEL deep-ocean hydrophone, pre-amp, and alkaline battery pack that are all contained in a 62 cm titanium pressure case. 

Figure S2 (123 KB pdf)
(a) Increase in depth in meters (red) and decrease in temperature in (°C) recorded by the RBR™ pressure sensor during the descent of the mooring at Challenger Deep. Maximum hydrostatic pressure recorded was 11,161.4 decibars, corresponding to a maximum depth of 10,829.7 m (the pressure sensor was 25 m above the seafloor). Temperature ranged from a maximum of 28.4°C at the sea surface to a minimum of 2.45°C at the seafloor. (b) Pressure record (depth) time series (with mean removed) from the RBR™ instrument during the two months of the total deployment. Tidal cycles are clearly visible, as is a gradual decrease over the first ~40 days of the deployment. (c) Power spectral density estimate of pressure record showing energy peaks associated with tidal fluctuations. The peak at left representing the neap cycle, and the semidiurnal peaks at 24 and 12 hours were recorded at 10,829.7 m ocean depth.

Figure S3 (672 KB pdf)
Examples of mid- and low-frequency cetacean sounds recorded on the Challenger Deep hydrophone. (a) Toothed whale/dolphin whistles, focused in the 8–11 kHz band with durations of 1–2 s. (b) Two-part, baleen whale tonal call with upper energy band at 400 Hz and low tone at 50 Hz, both parts of 1–2 s duration. These tones are the low-frequency part of a complex baleen call identified recently by Nieukirk et al. (2016). (c) Percent of the day the baleen vocalizations were recorded by the Challenger Deep hydrophone (manually selected by an analyst). 

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