The detection of very high energy neutrinos of galactic/extragalactic origin requires very large detectors and a large overburden as a shield against the background of cosmic ray muons. ANTARES is at present the largest (effective area ~0.05 km2) experiment currently under construction in the northern hemisphere. It is being built and installed at a depth of 2500m in the Mediterranean sea, near the Southern French coast, by a large European collaboration. A three-dimensional array of photomultipliers are used to detect the Cherenkov light emitted by neutrino-induced muons. The array, when completed, will consists of 12 lines each covering a vertical length of about 480 m and equipped with 75 photomultipliers arranged in triplets. The readout electronics is connected to an on-shore laboratory through a 42 km long electro-optical cable. The final detector design has been completed. An instrumented line (called MILOM) has been installed in the spring of 2005; the first string (Line 1) is in acquisition starting from February 2006, and the second (Line 2) from September 2006. The physics motivations of the experiment, the details of the construction and installation, together with preliminary results obtained using the MILOM and Line 1 are presented.