Three teenagers are dead and one is struggling to survive this morning after they were apparently overcome by carbon monoxide in the Wabasha caves in St. Paul. A fifth teenager managed to get out of the tunnel yesterday and told police that his friends were still inside. Officials say the group entered the cave through a small hole in the bluffs across the Mississippi River from downtown St. Paul. The complex of caves in the soft sandstone in that area can stretch for hundreds of feet. People exploring the area have died there before. An effort was made to improve safety by filling in the caves with debris from construction. But some experts believe that the effort may have made the caves more dangerous. Morning Edition host Cathy Wurzer spoke with Calvin Alexander, a professor at the University of Minnesota who studies geology.