[en] As a part of the confirming site investigations at Olkiluoto, Suomen Malmi Oy (Smoy) core drilled a 650.55 m deep drillhole with a diameter of 75.7 mm at Olkiluoto in May - June 2009. The identification number of the drillhole is OL-KR51. A set of monitoring measurements and samplings from the drilling and returning water was carried out during the drilling. Both the volume and the electric conductivity of the returning and drilling water were recorded. The drill rig was computer controlled and the computer recorded drilling parameters during drilling. The objective of the measurements was to obtain more information about bedrock and groundwater properties. Sodium fluorescein was used as a label agent in the drilling water. The total volume of the used drilling, washing and flushing water was 778.7 m³. The measured volume of the returning water in the drillhole was 570.3 m³. The deviation of the drillhole was measured with the deviation measuring instruments EMS and Maxibor II. Uniaxial compressive strength, Young's Modulus and Poisson's ratio were measured from the core samples. The average uniaxial compressive strength was 130.0 MPa, the average Young's Modulus was 57.0 GPa and the average Poisson's ratio was 0.23. The main rock types are veined and diatexitic gneisses, pegmatitic granite, K-feldspar porphyry and tonalitic-granodioritic-granitic gneiss. The average fracture frequency is 2.5 pcs/m and the average RQD value 95.2 %. 19 fractured zones were penetrated by the drillhole. (orig.)