Since May 2018, the island of Mayotte is subject to a massive seismic-volcanic crisis. Many earthquakes of magnitude greater than 5 occur off the coast towards the east and the island is deformed at record rates of more than 20 cm per year in connection with the emptying of a deep magmatic reservoir located in the oceanic domain, a few dozen of kilometers off the coastline. In order to document this crisis and better understand the risks, a national observation program has been set up. This program included the installation of onshore seismological and GNSS (GPS) stations, sea-bottom seismometers (OBS) as well as at-sea cruises program (MAYOBS) for deploying and recovering OBS and acquire geophysical data (bathymetry, bottom and water column imaging, sub-bottom profiles) to reveal traces of recent volcanic and seismic activity on the ocean floor. Dredging and CTD-Rosette measurements were also carried out. Within this context, we discovered that a major volcanic eruption was ongoing off the island with the birth of a new volcanic edifice of more than 820 m height, rising from the 3400-m ocean floor up to its summit located at 2580 m below the sea surface, involving considerable lava volumes (5km³, Feuillet et al., Nature, in review). Acoustic plumes of more than 2000m high, which origin is unknown for the moment, have also been observed rising from the summit of the volcano through the water column. Based on these initial results, the GeoFLAMME project aims at conducting a scientific multi-disciplinary research study documenting this major eruption by performing observations and in situ sampling (ROV, dredge, corers and CTD) and analyzes (petrology, chemistry, biochemistry) of rocks, fluids and water column combined with high resolution mapping, mosaic and 3D reconstruction of volcanic structures, tectonic features and fluid emissions (with ROV and AUV). Our main objective is to understand the context of this major volcanic crisis and, in particular the interactions between magmatism, tectonics and fluids circulation processes at the scale of the lithosphere on several spatial and temporal scales as well as the implications on the physical-chemical properties of the water column and the ecosystem evolution. This ambitious and novel project provides a combined study of solid, gas and liquid samples to unravel the entire chain of interactions between volcanic products, magmatic gases, seawater circulation and the water column, which is rarely done in the frame of a single project and in the context of a deep-sea eruption (active if still ongoing) or very recent. This project mobilizes close to 70 scientists (including forties onboard) from different disciplines (Geochemistry, Volcanology, Tectonics, Biology) and different French institutes and laboratories (IFREMER Brest, IPGP Paris, BRGM, ISTO Orleans, LMV Clermont, GEOPS, Paris South Orsay, LSCE, GET Toulouse).