The correct option is 12 km.
Explanation
Knowledge about the interior of the Earth is derived from two types of sources: direct sources (such as mining and deep ocean drilling projects) and indirect sources (such as seismic waves, gravity anomalies, and meteors). The extent of direct exploration is limited by the rapid increase in temperature and pressure with depth.
Detailed Analysis:
- Deepest Drilling Project: The deepest man-made borehole is the Kola Superdeep Borehole.
- Location: It is located in the Kola Peninsula, Russia, within the Arctic region.
- Depth: The drilling reached a depth of approximately 12 km.
- Comparison with Earth's Dimensions:
- 3-4 km: This is the approximate depth of the deepest underground mines (e.g., gold mines in South Africa). Going beyond this depth is technically difficult due to extreme heat.
- 6,378 km: This represents the approximate equatorial radius of the Earth (distance to the center).
Consequently, while 12 km is a significant engineering achievement, it penetrates only a minute fraction of the Earth's crust compared to the planet's total radius.
Key Takeaway:
The deepest point reached by human drilling is the Kola borehole at 12 km, which provides direct evidence regarding the composition of the upper crust.