What does this number measure?
We compare the reported vertical depth of SG-3, the Kola scientific borehole. David Smythe, designer and leader of the Kola-92 geophysical experiment, gives a vertical depth of 12 262 m reached in 1989. This depth starts from the borehole's local surface reference; the page does not specify a geodetic datum or an instrument tolerance. This dimension is neither an altitude below sea level, the cumulative length of the drilled branches nor a building height. The DFG-SPP ICDP programme page gives 12 261 m: this documented difference of one metre is not a statistical interval. The straight diagram compares the depth with Earth; it does not reproduce the borehole's actual path. Its width, line thickness and rock layers are illustrative, not measured dimensions of the well.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Historical reference from 1989: 12 262 m according to David Smythe; 12 261 m on the DFG-SPP ICDP programme page. This difference between publications is not presented as a measurement uncertainty. The source does not specify the tolerance, exact datum or all borehole deviations. Do not confuse vertical depth, the distance travelled by the tool and the total length of several branches.
Schematic geological cross-section, not a photograph. Only the vertical depth from the surface to the bottom is calibrated; borehole width, cross-section width and layers are illustrative, without a horizontal scale.