The Stenlille natural gas underground storage is located 70 km SE of Copenhagen and has been in operation since 1989. For safety reasons and to protect the environment it is necessary to monitor the storage carefully. Natural gas is being stored in an anticlinal structure with an expected gas storage capacity of about 3 billion Nm3 (volume under 'normal' conditions) in the upper Triassic Gassum Sandstone Formation 1500-1600 m below the surface; it replaces saline formation water. So far, nineteen deep wells have been drilled on and around the structure. The 300 m thick clay sequence of the Lower Jurassic Fjerritslev Formation above the gas storage reservoir has acted as an efficient seal, since no sign of gas leakage has been observed in the monitoring well located in a sand stringer 15 m above the gas reservoir. Other monitoring wells have been located in order to check for possible lateral escape of natural gas. A baseline study on naturally occurring hydrocarbons performed before the natural gas storage came into operation indicated the presence of only trace amounts hydrocarbon gases in the subsurface of the Stenlille area. Results of analysis by the headspace and sorbed gas methods on drill-cuttings suggest that low-temperature thermal generation of hydrocarbon gases (δ 13C 1: -47 to -42‰ δ 13C 2: -34 to -30‰) has taken place in organic-rich marine shale below 1300 m. Low concentrations of dissolved methane (<0.5 mg/l) of bacterial origin (δ 13C 1: -90 to -62‰) were found in shallow groundwater that is used for water supply in the Stenlille area. After the start of injection of natural gas in 1989 (C 1:C 2:C 3 = 91:6:2; δ 13C 1: -47‰), no increase in methane concentration and no higher hydrocarbon gases were observed during the regular analysis of groundwater from 10 shallow wells located above the underground natural gas storage. However, a sudden increase in dissolved methane concentration from 0.02 to 27 mg/l was measured in a 130 m deep observation well after a minor gas leakage had been detected at a new deep drilling into the natural gas storage in 1995. Nonetheless, no increase in methane was observed in shallow groundwater at the same locality. Occasional higher concentrations of dissolved methane (up to 15 mg/l) were encountered in shallow observation wells in low permeability layers. Stable isotope analyses (δ 13C 1: -69 to -52‰) and radiocarbon dating show that the gas does not originate from the underground gas storage because the methane was less than 300 years old, but it may have formed due to local microbial activity.