<p>“Floating reefs,” also known as “mega-blocks,” occur within kimberlite and non-kimberlite maar-diatreme volcanoes. In most cases, floating reefs consist of large accidental blocks derived from the pipe wall country rocks. Some other floating reefs comprise blocks of solidified cognate material from earlier emplacement phases. In addition, we include isolated entities of unconsolidated sediments derived from the host substrate or the crater floor as floating reefs, since they are of no economic value for mining. In general, floating reefs usually occur along the diatreme wall and may be intact or partially or completely fragmented. Within the diatreme, they have subsided from higher stratigraphic levels and can be found at all structural levels of a maar-diatreme volcano. Floating reefs can be hundreds of meters in size and may have subsided over 1000&#xa0;m. In both kimberlite and non-kimberlite diatremes, the presence of floating reefs suggests common growth processes. However, kimberlite diatremes lithify faster than their basaltic counterparts, which leads to a higher probability of cognate floating reefs in kimberlites. Subsidence of floating reefs is a consequence of mass transfer during an eruption when root zone material is ejected upward through the diatreme, allowing for subsequent collapse of diatreme tephra to fill the void explosion chamber. Compaction of the volcaniclastic diatreme fill allows further subsidence. The downward movement of the root zone during pipe growth also initiates lateral erosion and enlargement of the overlying diatreme. The upward migration of arcuate tear-off fronts along the pipe walls causes isolated blocks of wall rock material to subside into the diatreme which widens the diatreme and, ultimately, the maar crater. Floating reefs provide important clues on the growth process of diatremes and can also have a controlling effect on the facies architecture. On that basis, they can influence diamond distribution in a primary diamond mine.</p>