referring to fig1 , a cross section of an exemplary water heat accumulator in which the principles of the present invention may be advantageously practiced is illustrated generally . as illustrated , the exemplary water heat accumulator 17 is embedded completely in the soil 10 , thereby using the heat - storing properties of the soil 10 and the corresponding thermal insulating effect of ground heat . the upper part of water heat accumulator 17 has a cylindrical shape which extends into a lower part in shape of an inverted frustum . this geometry minimizes to the extent possible the skin surface of the heat accumulator 17 relative to its volume . water heat accumulator 17 could also use a square or polygonal layout . in practice the shape of water heat accumulator 17 will also be determined by its construction costs . a rectangular layout is possible , but deviates from the optimal geometry . water heat accumulator 17 is closed with a top cover , comprising protective membrane 3 , a pp film vapor barrier , and a correspondingly thick thermal insulation 5 . a foundation slab 11 , e . g . made of concrete , is located over the top cover of water heat accumulator 17 . foundation slab 11 may be placed on accompanying pe film 9 , and a parameter insulation 8 . the upper section of the accumulator 17 is waterproofed and thermally insulated . thermal insulation 5 is protected against moisture from the outside , e . g . caused by groundwater in the adjacent soil 10 , by means of a drain mat 13 . drain mat 13 is covered on both sides with filter membranes 12 . any existing moisture will be absorbed by a drainage pipe 14 , which is installed in a circumferential drainage trench filled with gravel 7 , and preferably fed into a rainwater management system . water collected in drainage pipe 14 may be used to fill heat accumulator 17 . foundation slab 11 may be the foundation of a building which may or may not have a basement . foundation slab 11 may also be the substructure for an outdoor area , street , or parking lot . until now , installing heat accumulators below buildings was not customary . this was because the cost - benefit ratio favored large or very large capacity water heat accumulators . generally , however , there is no free space available in crowded urban infrastructures to accommodate such large building projects , and accommodation of a superstructure , even over a gravel - water heat accumulator , has thus far been considered problematic in terms of both space requirements and technical feasibility . regardless of this fact , a water heat accumulator with a superstructure always has the advantage of very good thermal insulation from above and protection against surface water due to the building alone . water heat accumulator 17 further comprises pipes 2 for heat input and output and equipment for any required measurement and filling . the pipes 2 are fed through the accumulator &# 39 ; s cover and through the foundation slab 11 into the building by the shortest route . the wall feed - through points normally necessary for this purpose , also known as wall sleeves , are thermally insulated and waterproofed in relation to the building and / or the heat accumulator 17 . heat is transferred into ( input ) or withdrawn from ( output ) storage medium 16 through collector pipes and a heat transfer medium circulating therein . the double arrow shown in fig1 illustrates such reciprocal action for the individual collector pipe arrays . the sketch symbolically shows three collector pipe arrays independently installed in the accumulator at different levels / heights , which can store the heat with different temperatures in the accumulator and then withdraw it from this area . accordingly , the coldest temperature is at the bottom and the warmest temperature is at the top of water heat accumulator 17 . measurement and filling device 6 is provided to initially fill water heat accumulator 17 with water and to refill it later if necessary . measurement and filling device 6 measures the water level and the temperatures of the storage medium 16 and / or the water levels with different temperatures . storage medium 16 is made of a statically stable , i . e . solid , porous construction material that can be completely saturated with water . storage medium 16 may not require external waterproofing or a statically stable surrounding structure . this is because the porous , solid structure of the construction material absorbs the subsequently added water and retains it by means that include the capillary effect . this is advantageous over known gravel - water heat accumulators which would collapse and spill water into the surrounding soil if there were no supporting structure and waterproofing . storage medium 16 may be made of cement foam concrete construction material that is produced especially for this purpose . it may also be composed of pumice stone granulate with a cement - like binding agent . the construction material is mixed with water prior to processing as ready - made concrete or mixed - on - site concrete , and poured into the prepared pit in a liquid state . the pit may have a simple formwork on the sides , which can be removed after the construction material is hardened . this allows for installing the waterproofing films 12 , 13 and thermal insulation material 5 on the side walls of storage medium 16 after it has hardened . drain mat 13 with its associated filter membranes 12 and thermal insulation 5 may also be installed along the walls of the excavated pit before the construction material of storage medium 16 is poured . in this case a formwork is not required . whether or not to use additional formwork depends on the depth of the pit , the extent to which a banking of the pit walls is possible or desirable , and safety considerations . the lower area , i . e . the heat accumulator &# 39 ; s base cup , may not need to be additionally waterproofed prior to the poring of the construction material , in which case the pit remains in its raw condition . waterproofing and thermal insulation of the accumulator &# 39 ; s base are optional . a concrete base before pouring the construction material forming the heat storage medium 16 is never required . simultaneous with the pouring of the construction material forming storage medium 16 , the collector pipes which are connected to input and output pipes 2 and the pipe with the corresponding sensors of the measurement and filling device 6 will also be embedded in the concrete . once the binding material and / or the concrete has hardened , the solid body of the storage medium 16 can be stripped and clad on the sides with the additional films , membranes , and insulating materials . the accumulator 17 is then filled in once again around its sides with soil . inherently , an additional concrete shell 15 is automatically formed seamlessly with the construction material in the soil around the accumulator without additional effort . this creates a certain additional external stabilization and waterproofing for the storage medium 16 . this is based on the fact that when filling in the construction material , outwardly saturated binding agent penetrates into the adjacent stone structure of the pit , and after hardening forms a solid concrete mass together with the soil . except for a narrow upper residual height , the entire accumulator is now completely filled out with this solid construction material in a formfitting manner . the residual height is then filled out with gravel 7 before the cover with the pipe pass - throughs is installed and sealed . the accumulator can then be filled with water . there are hardly any risks of construction defects with this technology because the storage medium 16 is self - supporting and to a large extent self - sealing and water retaining . it is also additionally supported by the cap - shaped sealing of the storage medium 16 in the area of the cover and the sides , which in the event of water loss creates a vacuum below the cap , and thus in the storage medium . this vacuum can also be measured with the measurement and filling device 6 . in addition to a moisture sensor and / or water level indicator , the height of the vacuum can also be a parameter for the water level 1 in the accumulator . this should optimally adjust to the height of the gravel layer 7 , and can be supplemented if necessary by means of the filling device 6 . filling the accumulator with water certainly takes longer than is the case with a gravel - water heat accumulator , because the construction material can absorb the water only slowly through the pores , capillaries , and its hygroscopic properties . this is not a problem , however , because there is no need to begin operations before the building that will use the heat is built above the accumulator . while the present invention has been described with reference to exemplary embodiments , it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but , on the contrary , is intended to cover numerous other modifications , substitutions , variations and broad equivalent arrangements that are included within the spirit and scope of the following claims .