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."