"We can detect sunspots at a depth of 60,000 kilometers, but if we try to detect them deeper or closer to the surface, the travel-time shift becomes weaker," Ilonidis says.

"We can detect sunspots at a depth of 60, 000 kilometers, but if we try to detect them deeper or closer to the surface, the travel-time shift becomes weaker," Ilonidis says.

This shift is due to both improved safety standards and technology as well a move away from underground mines towards surface mining and controversial mountaintop-removal mining.