We observed the dynamics of dry patches underneath massive bubbles during the pool boiling of saturated water under atmospheric pressure, and measured the associated temperature distribution. We synchronized the observations both spatially and temporally using high-speed total reflection and infrared thermometry techniques. The observations presented in this paper provide evidence that the critical heat flux phenomenon is triggered during the rewetting of large dry patches with periphery temperatures that are much lower than minimum film boiling temperature, so-called Leidenfrost point. As the liquid meniscus advanced toward the dry patch, numerous secondary bubbles nucleated and impeded the flow of liquid toward the dry patch. This prevented the liquid from rewetting the dry patch. The key physical mechanism for triggering CHF is initiated when the line density of the secondary bubbles nucleating at the periphery of a shrinking dry patch underneath a departing mushroom bubble exceeds a critical value. These bubbles completely block the liquid inflow into the dry patch and prevent rewetting, eventually causing the dry patch to expand irreversibly. We used our experimental data to determine an empirical value of the critical line nucleation site density. (C) 2018 Elsevier Ltd. All rights reserved.