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Lifou is an atoll uplifted to a near 100 m height near the Vanuatu Trench. Extensive karstification is recorded in the outer coral rim and to a lesser extend in the lagoon type limestones of the inner island. The orientations found in the longest karstic networks correspond rather well to the main fracture directions, but near coasts, the galleries are controlled by the paleo-shoreline, suggesting that mixing corrosion might be involved during cave formation. The freshwater level and temperature, recorded during one year inside two caves present smooth variations, if tidal components near 12 and 24 h periods are removed, and are poorly correlated with rain. Comparison with data recorded in a continental karst (Baget, Pyrenees) and in the chalk of the Paris Basin indicates that Lifou lacks an organized channel network able to collect and transfer quickly rainwater toward the phreatic zone.