12 Kilometers Deep: What the Kola Borehole Revealed About Earth's Crust
From 1970 to the early 1990s the Kola Superdeep Borehole reached 12,262 meters, but the bottom rock heated and deformed plastically, sealing the bore rather than revealing Earth's center; importantly, this depth is vertical, not the bore's total length, since the drill path zigzagged with sidetracks. The project yielded thousands of cores and refined crustal models, showing a continuous transition into ancient granite‑gneiss and indicating the Moho boundary lies well beyond the hole. Temperatures at depth reached roughly 180–212 C, and the operation faced extreme wear and logistical challenges; no borehole has crossed the Moho or sampled mantle, and modern ultradeep drilling now targets crustal structure and geothermal topics rather than a journey to the core.