An unusual combination of conditions in Amsterdam, the Netherlands, required that a method be developed for building a structure of a virtually impermeable and crack-free concrete and sinking it into the ground pneumatically. Such a method was used to build a Metro intended to relieve that city's congested inner-city traffic situation. Most of the land under Amsterdam was put there by man; sand is the main component to a depth of three yards or more. Below this are alternating layers of coarse sand, peat and clay. The water table is encountered only a little over a yard (one meter) below street level, and it must stay as precisely that depth to protect the piles on which the prewar houses are built. The tops of the piles begin to deteriorate if they are allowed to dry out. These conditions, as well as those of cost, noise level and traffic disruption led to the choice of the pneumatic caisson method of construction for most of the underground sections. The pneumatic caisson method employs a reversal of normal tunnel construction technique whereby a trench is excavated and concrete is placed directly into its final position. The caissons are constructed with a sort of false bottom. Each rests on ridges that extend downward from its floor and create a hollow area beneath. Within the hollow area are powerful water jets to wash away the ground on which the caisson is resting while high pressure air is pumped into this chamber to keep out ground water. As the diluted water-sand mixture is pumped to the surface and away, the caisson sinks slowly into the ground. A gap of about 19 inches remains between adjacent caissons in position below ground; this of course must be filled to provide continuous tunnel. Filling is done by means of a cryogenic method devised by the French. Liquid nitrogen is sent through pipes placed in the gap to freeze the soil so it can be mechanically smoothed and excavated. Waterproof insulation is placed against the frozen surface. A mold is constructed flush with the tunnel walls and a concrete mix made with Type B blast furnace cement is pumped in at a number of points. On the other side a foam rubber layer separates the joint concrete from the caisson and allows for movement. A layer of fill concrete is then placed on the tunnel floor.