The phrase "deep tunnels" refers to underground paths or passageways that are located far below the surface. These tunnels are typically used for various purposes such as transportation, utility services, or even secret hideouts. Full definition
However, in - train cooling is of limited usefulness, particularly in deeper tunnels. (newscientist.com)
Other dive sites range from deep tunnels to sea mounts, huge coral formations and marine life. (stmaartencruiseexcursions.com)
On the ground, Lück says, the arms of laser interferometers are unlikely to ever exceed 50 kilometers — past that, seismic noise, Earth's curvature, imperfect optics and the great expense of digging deep tunnels would outweigh any conceivable scientific gains. (scientificamerican.com)