Belharra is a wave energy hotspot located on the French Basque coast. Here, unusually large waves occasionally break over a 15-meter deep seamount, producing world-class conditions for big wave surfing. This does not occur often, even though the region frequently encounters highly energetic swells, especially during winter. This work attempts to better understand the underlying processes leading to extreme waves at this location. A combination of spectral and phase-resolving wave models enables analysis of how the incident swell conditions and tide level affect the wave field at both regional and local scales. We carry out a detailed wave-by-wave investigation of the local conditions for a wide range of plausible scenarios. Qualitative comparisons are drawn with data from the trajectory of a surfer riding waves at Belharra during a big swell event. Regional and local features in the bathymetry appear to govern the concentration of swell energy at this spot. Long-period swells from directions around 300° - 315° generate the most energetic local conditions. Lower tide levels lead to greater breaking intensities, albeit with weaker wave amplifications.