When we talk about a sink hole, we are talking about a caved in underground cavity or a caved in and grown over tunnel entrance which can be seen when looking at the landscape even today.  Sink holes can be found in the Montafon, especially on the Kristberg and they point to early mining.  Some of them have been studied by the archaeologists who proved that mining was done here.  You can receive detailed information about mining in early times here.