Unusual speleothems resembling giant mushrooms occur in Cueva Grande de Santa
Catalina, Cuba. Although these mineral buildups are considered a natural heritage, their
composition and formation mechanism remain poorly understood. Here we characterize
their morphology and mineralogy and present a model for their genesis. We propose that
the mushrooms, which are mainly comprised of calcite and aragonite, formed during four
different phases within an evolving cave environment. The stipe of the mushroom is an
assemblage of three well-known speleothems: a stalagmite surrounded by calcite rafts
that were subsequently encrusted by cave clouds (mammillaries). More peculiar is the
cap of the mushroom, which is morphologically similar to cerebroid stromatolites and
thrombolites of microbial origin occurring in marine environments. Scanning electron
microscopy (SEM) investigations of this last unit revealed the presence of fossilized
extracellular polymeric substances (EPS)—the constituents of biofilms and microbial
mats. These organic microstructures are mineralized with Ca-carbonate, suggesting that
the mushroom cap formed through a microbially-influenced mineralization process. The
existence of cerebroid Ca-carbonate buildups forming in dark caves (i.e., in the absence
of phototrophs) has interesting implications for the study of fossil microbialites preserved
in ancient rocks, which are today considered as one of the earliest evidence for life on
Earth.

During open-pit quarrying and related lowering of groundwater level in the gypsum karst aquifer (since 1950), large cave Zoloushka became accessible for direct exploration, in which considerable geochemical transformations of environment occurred, accompanied by the formation of specific deposits, as well as by burst of microbial activity. Among microorganisms, some of the most active were various iron bacteria. Microbial activity has resulted in precipitation of black and red biochemical formations – microbialites (coatings, crusts, films, stalactites, stalagmites, etc.), which cover walls and floors of cave passages. Most interesting among the microbialites are iron-rich colonial formations of various shapes (stalagmite-like, tube-like, coral-like, etc.) formed by yet unidentified fungi-like microorganisms which likely are new to science. In this paper, we characterize occurrence and morphology of the colonial aggregates, morphology and chemical composition of microorganisms  and develop working hypotheses of their identification.