Sealing off the surrounding groundwater is a factor of decisive importance when excavating pits. Any inflow of water or soil must be reliably prevented. Special precautions are needed to protect personnel and equipment against such "surprises". Sheet piling is one method used; injection seals are another, or the groundwater level can also be lowered. Often enough, these precautions do not suffice on account of adverse soil conditions. Special solutions are needed in such cases, such as freezing the soil with liquid nitrogen. The risk of collapsing soil and rocks is a serious danger in large excavations and tunneling operations. There are many methods of stabilization which are often very costly but do not always solve all of the problems. In comparison, soil freezing with liquid nitrogen is extremely simple and particularly safe. It is the only process whose effectiveness can be measured objectively. The medium, which has a temperature of minus 196 degrees Celsius, turns the ground around the construction site into a stable block of ice that is impermeable to water.