We observed the global boiling structure and dynamic behavior of dry areas in a synchronized manner to identify the critical heat flux (CHF) triggering mechanism in a horizontal pool boiling of saturated water. A transparent Indium Tin Oxide (ITO) heating surface was used to accommodate a total reflection technique. The total reflection images captured the detailed processes of the generation of discrete dry spots, the formation and rewetting of large dry patches, and the irreversible expansion of the dry patch which led to the occurrence of the CHF. Contrary to the common postulation that a thin liquid film exists stably under a coalesced massive bubble, the base of hovering massive bubbles was almost dry at more than 10% below the CHF condition. The key element that determines the occurrence of the CHF was the production of the residual dry patch and consecutive re-expansion of the residual dry patch owing to the enhanced bubble nucleation activity with an increase in wall superheat, rather than the complete dryness of a boiling surface under a hovering massive bubble. The dry area fraction of the present water boiling test was similar to that of the R-113 boiling test in the literature in spite of significant differences in the wettability and physical properties.