  New generations of gravity wave detectors require unprecedented levels of
vibration isolation. This paper presents the final design of the vibration
isolation and positioning platform used in Advanced LIGO to support the
interferometers core optics. This five-ton two-and-half-meter wide system
operates in ultra-high vacuum. It features two stages of isolation mounted in
series. The stages are imbricated to reduce the overall height. Each stage
provides isolation in all directions of translation and rotation. The system is
instrumented with a unique combination of low noise relative and inertial
sensors. The active control provides isolation from 0.1 Hz to 30 Hz. It brings
the platform motion down to 10^(-11) m/Hz^(0.5) at 1 Hz. Active and passive
isolation combine to bring the platform motion below 10^(-12) m/Hz^(0.5) at 10
Hz. The passive isolation lowers the motion below 10^(-13) m/Hz^(0.5) at 100
Hz. The paper describes how the platform has been engineered not only to meet
the isolation requirements, but also to permit the construction, testing, and
commissioning process of the fifteen units needed for Advanced LIGO
observatories.
