The Genesis Flood must have produced drastic geological changes involving extremely energetic processes which must also have generated an enormous heat load. Yet since the inhabitants of the Ark and many aquatic creatures survived the Flood, the heat must have been removed without raising temperatures beyond biological endurance limits. This is the first in a series of papers intended to identify and, where possible, to quantify the most important sources of Flood heat. Here we review the thermal state of the earth's mantle, crust and oceans, both now and in the recent geological past, and take note of mantle heterogeneities as tracers of its tectonic history. Extensive temperature records based on the oxygen-18 content of marine fossil shells indicate geologically recent ocean floor temperatures not exceeding 13ºC; all models of the Flood and its aftermath need to take this limit into account. A recent attempt to model ocean floor cooling within a biblical chronology by invoking a transient subsurface heat sink along with rapid oceanic plate movement was unsuccessful, suggesting that important hitherto-neglected geophysical effects may need to be included in future. Inventories of heat-producing radioactive elements estimated from rock compositions and from geoneutrino measurements are in broad agreement, lending confidence to the anticipated process of estimating the heat load due to accelerated nuclear decay during the Flood. Preliminary estimates for granite suggest that this radiogenic heat load would be overwhelming without the operation of a powerful in-situ cooling mechanism which has yet to be identified.