The big heavy milk bottle wasn't as easy to drop in as might first appear. Here's a comment from the ASLA awards announcement:
"Throughout the project, construction techniques were employed to address the need for durability on a constantly shifting site. Expanded polystyrene foam (Geofoam) was used as fill below the finished plaza to offset the weight of the new landscape and to prevent further settlement. Steel piles driven into the native bedrock were installed to support heavier elements like the marble boulders and the Milk Bottle. A waterproof liner system was developed to contain the roots of the willow trees while keeping the saline ground water from contaminating the planting soils."