The Calanques stretch 20 kilometers between Marseille and Cassis, a succession of white limestone cliffs plunging into Mediterranean water so clear the seabed remains visible at twelve meters. Created in 2012, Calanques National Park protects 8,500 hectares of terrestrial terrain and 43,500 hectares of marine area, making it Europe's only peri-urban national park to span both land and sea. The word calanque derives from the Provençal calanco, meaning steep-sided inlet, a geological feature born from the flooding of river valleys at the end of the last ice age. What remains today are narrow fjord-like corridors with vertical walls reaching 400 meters, sculpted by millennia of wind, salt, and tectonic shift. Cassis, the easternmost fishing port along this coast, has served as the principal departure point for calanques boat tour with swimming from Cassis since the first motorized vessels began running excursions in the 1920s. The town's sheltered harbor sits beneath Cap Canaille, continental Europe's highest sea cliff at 394 meters, a rust-orange escarpment that signals the transition from limestone to sandstone. Boat access remains the most practical means of reaching the calanques' inner coves—Port-Miou, Port-Pin, and En-Vau—where footpaths either do not exist or require three-hour descents over loose scree. Swimming stops in these protected waters reveal marine ecosystems that include Posidonia oceanica meadows, purple sea urchins, and occasional loggerhead turtles, all thriving under strict conservation protocols that prohibit anchoring in seagrass zones. The calanques boat tour with swimming from Cassis operates year-round, though departure frequency peaks between April and October when the mistral wind moderates and water temperature climbs above 18°C. Electric boats and catamarans now dominate the harbor's tour fleet, replacing diesel vessels to meet the national park's noise and emission standards introduced in 2019. Each tour type follows designated marine corridors established by park authorities to minimize disturbance to nesting seabirds on the cliff faces, including Bonelli's eagles and peregrine falcons. The limestone itself contains fossils from the Urgonian formation, 120-million-year-old remnants of a tropical reef system that once covered southern France when the region lay submerged beneath the Tethys Ocean. On the water, that ancient geology reads as a study in negative space—shadow, light, and the sapphire gradient where depth drops from three meters to thirty in a single boat length.