Passive acoustic monitoring systems that can be deployed for a long period of time (few months) is a needed for the work NURC is doing in the field of marine mammals risk mitigation but also would be very suitable for the port protection project where the deployment of long-term acoustic systems could be considered/required for the detection of threats such as divers and small surface vessels. Fortunately, advances in low-power and high-data-capacity consumer computer technology during the past decade have facilitated the autonomous recording of sounds from marine environment, and in particular for marine mammals over long periods of time. Different systems, with different characteristics, currently exist but are unfortunately not always commercially available and/or not exactly meeting NURC requirements. For that reason, NURC decided, in 2010, to develop its own system, called HYDRA for High Yield Data Recording Array, for enhancing the performance of current available systems in combining and extending the advantages of each of them such as low cost, large bandwidth, high dynamic range, long storage and large autonomy. This paper will give an exhaustive description of the system and compared it to current existing systems. The developed system is capable of simultaneously recording up to 8 acoustic channels, with 24 bits resolution, sampled up to 144 kHz and will have 53 days autonomy and be able to record today up to 2 TBytes of data. The system will be capable of being deployed up to 2000 meters water depth. Complementary to the recording capability, the proposed system has been enhanced by an onboard processing capability that will allow performing embedded real-time detection and classification of acoustic events.