The book mentions that at one point when they were investigating the chimney, the US operatives discovered a tunnel that extended past the embassy grounds. A t

The book mentions that at one point when they were investigating the chimney, the US operatives discovered a tunnel that extended past the embassy grounds. A team was doing a harrowing journey down the tunnel, uncertain of when they'd hit Soviet territory, and they got their sights on some Soviet operatives at the other end of the tunnel. Shortly after that the tunnel was mysteriously rendered inoperable by some sort of cave-in.