| Grand Som | |
| Elevation M: | 2026 |
| Prominence M: | 887 |
| Prominence Ref: | [1] |
| Range: | Chartreuse Mountains |
| Location: | Isère, France |
| Map: | Alps |
| Coordinates: | 45.3706°N 5.8119°W |
The Grand Som is a mountain of the Chartreuse Mountains in the French Prealps, in Isère, France. It has a long ridge, unevenly sharp, overlooking the Petit Som (1,772m), the Col de la Ruchère northwest and the Grande Chartreuse monastery below the cliffs. The summit has a cross and two relief tables. It lies in the Parc Naturel Régional de la Chartreuse.
Many chasms are present on the massif: Trou Lisse à Combonne (−303 m), puits de l'Écho (−396 m), gouffre Roland (−481 m), gouffre de Mauvernay (−507 m), gouffre des Aures (−512 m),, but the most important network is the puits Francis (1,565 m) or of 723 meters deep for 6,836 m of development. Discovered in July 1966 by the caving club of La Tronche (FLT), the siphon at (−688 m) was reached on August 23, 1967. Upper entries were then found.[2]