I usually use this expression when traveling through the heart of a city, usually by car, and usually ending with the expression ‘of all things’ in a Irish brough <sp>.
What is the actual meaning of the saying?
Images of a swordsman fighting a dragon and actually tunneling through it’s belly and walking through comes to mind. But then agan it may be someone going through the mouth and eventually out the anus (if dragons have anuses that is).
Am I even close?
The first time I heard it, it was the title of a book by convicted murderer Jack Henry Abbott (“In the Belly of the Beast”). It was composed of his letters from prison to Norman Mailer.
I always assumed it referred to Jonah and the whale but I’m having a tought time finding confirmation.
Doesn’t it refer to the Biblical story of Jonah being swallowed for three days in the belly of a great beast? There are lots of religious sites that talk about this story, and many of them point out that the word “whale” is a mistranslation. Fish is the word usually used, but beast conveys the metaphorical meaning even better.
The original expression “in the belly of the beast” is a reference to biblical story of Jonah (though “beast” is not used in the scripture, it is a popular reference):
So there is more than one way to get out of the belly of a beast.
I’m so glad I’ve always heard “IN the belly of the beast,” not “THROUGH.”
Well, it could be a reference to the Quick-digesting Gink.
Of the handful of times I’ve heard the phrase (mostly from vets), I’ve heard the phrase in context as a euphemism for “the darkest hour”, “the deepest sh*t” or some form of the worst possible moment of any bad situation you’re in that usually has an ending.
Cases in point: I was once talking to a Vietnam Vet who pulled “tunnel rat” duty, and I remember him mentioning sweeping the tunnels as being in “the belly of the beast”. Another time, I remember talking to my neighbor who flew in WWII as an escort fighter with the B-17s over Germany–again, when he got into swarms of Messerschmitt fighters and AAA flak, he called it flying into “the belly of the beast”.
So, I’ve always heard/equated it as being in the most dangerous predicament for the particular moment or event.
Tripler
But then again, context doesn’t apply. There’s no “text” when speaking. 
I had always taken it to mean travelling through (or arriving “in”) the heart of a bad or dangerous area in the context of having no other alternative, such as on the front lines of a war, or in the general area of the war-torn area. In another similar context, one could think of it to mean being in the most dangerous and critical of situations, such as being a cop working undercover infiltrating a mob family – being in that crucual position of having gained the mob’s trust and being a key part of its operations, would be considered being “in the belly of the beast.”