A Roman poet described a Celtic sacred grove as a place locals were too frightened to enter, and that single hostile account still does a large share of the work in shaping what people imagine these groves looked like.
One especially vivid, especially biased description
The Roman poet Lucan, writing about a grove near Massilia, modern Marseille, describes trees hung with grim offerings and a site so feared that even the resident priest avoided approaching it at certain times, an image often quoted precisely because of its vividness rather than because independent sources confirm its accuracy, and modern scholars generally treat it as heightened literary and political rhetoric rather than sober ethnographic description.
Cut down as a form of conquest
Roman military and administrative campaigns in Gaul and elsewhere included deliberate destruction of sacred groves as an instrument of pacification, removing physical sites of local religious authority and communal gathering as part of subduing a conquered population, a practice that treated the groves' destruction as politically useful rather than merely incidental to warfare.
As this entry's companion article covers
The reliance on place-name evidence containing the root "nemeton" across a wide span of formerly Celtic-speaking Europe, as one of the few surviving traces of where such groves once stood, is addressed directly in this entry's companion article rather than repeated here.