A deep research borehole was drilled in 2004–2005 in eastern Finland by the Outokumpu Deep Drilling Project of the Geological Survey of Finland (GTK). The 2516 m deep hole was drilled into a Palaeoproterozoic metasedimentary, igneous and ophiolite-related sequence of rocks in a classical ore province with massive Cu-Co-Zn sulphide deposits. The hole is the deepest ever drilled in Finland. The Outokumpu Deep Drilling Project was carried out in an international framework, partly supported by the International Continental Scientific Drilling Program (ICDP). This Special Paper comprises fifteen articles discussing the deep hole results in terms of petrology, metamorphism, hydrogeology, seismic structures, seismic rock properties, density and magnetic properties, magnetic down-hole logging results, hydrogeology, fluid inclusions, geothermics, the deep biosphere, graphite- and sulphide-bearing schists and organic compounds in them.