Complex sentence: The tunnels are very unstable as contrary to popular belief most were not boared into solid granite, but loose shale, this is evident due to the large numbers of roof collapses in the incompleted unlined tunnels.
Split into simpler sentences: The tunnels are very unstable as contrary to popular belief most were not bored into solid granite, but loose shale. This is evident due to the large numbers of roof collapses in the incompleted unlined tunnels.