 Construction on the Brookville Tunnel , or the " middle " or " second tunnel " as it was then called , was begun sometime between 1849 and 1851 . The earth encountered at Brookville was of a much weaker quality than at the Greenwood Tunnel , rendering the digging of the tunnel both an extremely dangerous and difficult task . Although the eastern section of the ridge was composed of firm and sturdy rock , the majority of the tunnel passed through a mixture of frail slate that was formed at a steep angle , soapstone , and clay . When exposed to air , this mixture would often disintegrate causing large amounts of earth to fall into the tunnel bore . On two occasions during the tunnel 's construction , though great care was taken to provide temporary support , major cave @-@ ins occurred forming craters on top of the ridge , as much as 150 feet ( 46 m ) above the tunnel . Another slide occurred on the outside of the western portal that blocked the entrance to the tunnel and prevented workers from repairing the damage from the two internal cave @-@ ins . Rocks often fell down upon the timbers under which the men worked , creating sounds described as " resembling that of distant thunder " . The weakness of the earth , especially in the western sections , necessitated the implementation of a strong arch throughout approximately 500 feet ( 150 m ) of the tunnel and a durable brick lining . Crozet described the nature of the work on the tunnel in an 1854 letter : 
