The redistribution of matter in the deep sea depends on water-flow currents and turbulent exchange, for which breaking internal waves are an important source. As internal waves and turbulence are essentially three-dimensional (3D), their dynamical development should ideally be studied in a volume of seawater. However, this is seldom done in the ocean where 1D observations along a single vertical line are already difficult. We present the design, construction, and successful deployment of a half-cubic-hectometer (480 000 m3) 3D-T mooring array holding 2925 high-resolution temperature sensors to study weakly density-stratified waters of the 2500-m-deep western Mediterranean. The stand-alone array samples temperature at a rate of 0.5 Hz, with precision < 0.5 mK, noise level < 0.1 mK, and expected endurance of 3 years. The independent sensors are synchronized inductively every 4 h to a single standard clock. The array consists of 45 vertical lines 125 m long at 9.5 m horizontally from their nearest neighbor. Each line is held under tension of 1.3 kN by a buoyancy element that is released chemically 1 week after deployment. All fold-up lines are attached to a grid of cables that is tensioned in a 70-m-diameter ring of steel tubes. The array is built up in harbor waters, with air filling the steel tubes for floatation. The flat-form array is towed to the mooring site under favorable sea-state conditions. By opening valves in the steel tubes, the array is sunk and its free fall is controlled by a custom-made drag parachute reducing the average sinking speed to 1.3 m s−1 and providing smooth horizontal landing on the flat seafloor.