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.