When you dig tunels you can find some ruines, artefacts and old monuments. After completing dig'em up you can claim these for unloking some levels or to gain some skills.
old roman armor ? after a century under the soil, any metal become unusable... here you'd like to use an armor that at least stayed in earth for 500 years.... ?
I mean to find ruins and monuments to claim for upgrading some skills or levels! Not for personal use. If you dig a tunel and find an ancient sword you can use it as a sacrifice to increase your monument radius. Or if you find a ruin of an ancient stone building after you complete dig it up you can claim on this to increase your combat skills.