besides the cave entrances we can find on the surface which sometimes lead deep into earth, its based on luck to find new caves if you digged down into the rock.
We already have the metal detector for example to find hidden ore deposits. My idea is a similar tool to find hidden fluctuations of density underground. In other words holes, caves and other structures which are distinguishable from massive rock. Like echo probing (sonar) or radar.
For the explorers among us that might be a nice new tool.^^