Actually, if I recall, the tunnel caved in and Cassie was thought "dead". But before that, she was a foster kid to the Capwells or something (been so long) and teen Mason was jealous of the attention she received. They argued in the tunnel, and there was some cave in and Cassie "died", I think. (I think this was one of the reasons Mason began drinking.)
And the Four Orphans (Craig: John Callahan, Steven: John O'Hurley, the Derek actor [heh], and Ethan: Leigh J. McCloskey) I THINK suspected Mason had intentionally murdered Cassie - or, at least, Craig did...and hated Mason for it and tried to seek revenge.
And teen Mason in the flashbacks with Cassie in the tunnel (around '90) was played by none-other than a then-virtually unknown Leonardo DiCaprio [TL was Mason then, so a blonde Mason fit! ]!