This dataset presents EM304 multibeam echosounder acoustic data, collected over BlackMud Canyon on the continental shelf edge in Northern Bay of Biscay. Data files include bathymetry, seabottom backscatter, and water column acoustic data in echosounder kmall format, recorded onboard R/V Thalassa during EM304 Sea Acceptance Test on 08/09/2018 and 09/09/2018. Bathymetry ranges from 200m to 4200m depth. Kmall files can be displayed and processed using e.g. the GLOBE freeware provided by Ifremer at: https://www.flotteoceanographique.fr/Nos-moyens/Logiciels-de-la-flotte/Analyse-et-traitement-de-l-information/GLOBE.