Do you know which book i'm taking about?

so, there's this one book that i read a very very long time ago, but i can't remember the title of, nor much of the plot. i know the loch ness monster was involved, but not in a big way, there was an underground laboratory of a mad scientist which the protagonists go to in search of a cure,and the ending's sad, because i think the tunnel collapses and the both die, or one of them ho needed the cure doesn't get the meds in time, or both. help? please?