  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 one 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 build-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.
