The Lion mooring is located in the center of the Gulf of Lion where one of the largest open sea convection happens in the winter period. This open sea convection is an active vertical mixing that concerns the whole northern Mediterranean and can reach great deep in the Gulf of Lion (where it may even reach the bottom ~ 2500m) due to the Mistral and Tramontane, cold and dry winds. This is the phenomenon of dense water formation which is the only mechanism for the ventilation of the deep waters of the entire western Mediterranean. It was observed that the deep waters can be formed 1) in a preconditioned open sea area and 2) on the shelf with a subsequent cascading down the slope. There are therefore two modes whose interannual monitoring is necessary to understand the evolution of Mediterranean deep waters and validate ocean numerical models of the region. The LION mooring is deployed and maintained since 2007. It aims to observe the winter dense water formation affecting the north-western Mediterranean Sea water circulation and deep-sea ecosystem (physical data). The mooring is deployed near the ODAS meteorological surface buoy (ODAS Lion, Meteo France) and integrated in the MOOSE network since 2010. It is part of the FIXO3 and OceanSites systems.