Mesoscale eddies are central to how the ocean transports and mixes heat, water masses, and biogeochemical tracers, yet their three-dimensional interior structure and the processes acting at their margins remain undersampled by conventional observations. Here we combine coincident multi-channel seismic reflection imaging, vessel-mounted ADCP, satellite altimetry, and in situ hydrography from the MGL1701 cruise (early 2017) to resolve a subsurface anticyclonic eddy in the eastern South Pacific. The surface envelope from the Mesoscale Eddy Trajectory Atlas version 3.2 delayed-time product (META3.2 DT) was horizontally asymmetric, while the subsurface core was offset from the surface center and displayed a persistent, tilted dual-core structure, with an Equatorial Subsurface Water (ESSW)-like upper core and an Antarctic Intermediate Water (AAIW)-like lower core. The velocity field was strongly flank-dependent, with opposite-flank extremum-speed asymmetry of 16–27% and section-based apparent inclinations of 0.12°–0.27°. High-resolution seismic sections revealed steep, edge-confined reflector bands interpreted as finescale thermohaline filaments, and the turbulent kinetic energy dissipation rate was approximately one to two orders of magnitude higher at the eddy margin than in the core—yet decoupled from the smoothed Rossby-number and background-shear fields, implicating finer-scale processes. These results depict a relatively coherent, hydrographically differentiated interior surrounded by a finite-width, mixing-prone margin that governs leakage and exchange. Because the eddy retains and carries ESSW within the regional oxygen minimum zone, this margin controls how effectively such features transport tracer- and oxygen-anomalous water, while surface tracking alone misrepresents the subsurface core and biases coherent-transport estimates. Seismic oceanography thus offers a powerful tool for resolving subsurface (sub)mesoscale structure and boundary mixing relevant to ocean–climate interactions.