The tunnel is built in successive sections by means of a machine M suitable for operating at the surface and in immersion that is displaced in the water on the sub-sea soil, this machine comprising a fluid-tight working space (6) for accommodating the personnel and equipment required for construction, this space having a rearward-facing opening for building and erecting a section at the rear of the machine and the machine comprising in its forwared section a ballastable chamber (7) equipped with means (21-23) for preparing and grading the soil in readiness for erection of a section.Application to construction of a tunnel on the bottom of a body of water.