Its name literally means 'bathed bread' — the round loaf is deliberately soaked in olive oil and vinegar until it's closer to salad than sandwich.
Pan bagnat packs a round crusty roll with tuna, hard-boiled egg, tomato, olives, anchovies, and raw vegetables, all doused in olive oil and pressed under weight so the bread soaks up the dressing. Market stalls sell it pre-made and wrapped for the beach.