Cruise SO173¬1 SUBDUCTION¬II of the SFB¬574 was carried out in July 2003 using RV SONNE offshore Costa Rica and Nicaragua. Beyond other targets mound structures were observed using the GEOMAR deep towed multichannel seismic streamer. Besides a survey area offshore Nicaragua (Breitzke et al.) sites of mound structures off Costa Rica were revisited. Previous expeditions using RV SONNE and RV METEOR already studied Mound Culebra, a prominent mound structure of 150 m height, with multichannel and wide angle seismic profiling. This time multichannel deep towed seismic data as well as sidescan sonar data were recorded along 8 profiles which are offset by approximate 600 m. Additional lines with 1000 m offset cover the surrounding area where additional seafloor highs (additional mound structures) were identified from swath bathymetry. A BSR reflection is visible on all reflection seismic profiles with variable intensity. Within single fold seismic sections the BSR display becomes very weak underneath Mound Culebra, however final data processing has to proof if it can be traced continuously underneath the mound. ENE of Mound Culebra another seafloor high could be identified as a mound structure within seismic records. Again BSR patterns are clearly visible to both sides. An overview of the data and first results will be presented.