Abstract: The invention relates to the immersion at a great depth of an offshore weight-structure comprising a concrete base having two multicellular compartments, the one being central and self-resistant and the other being peripheral and subjected to compensating pressures, at least one multitubular pylon fixed to the said base and having a working area installed above it out of the water, and an auxiliary float co-operating with each pylon. The invention is characterized in that ballasting is effected by partial filling of the said self-resistant central compartment to enable the beginning of the immersion to be effected and that the actual immersion is then effected by the action of the said auxiliary float having an elongated cylindrical shape and being partly filled and whose relative position in relation to the said base is varied by mechanical guiding means.