It looks like an alien flower, but this spiny tentacle is actually a geological formation. The most awe-inspiring specimens are found in Brazil's limestone cave Caverna da Torrinha.
This photograph was taken by Coen Wubbels, and you can see more of his incredible shots inside the Brazilian caves where these rock formations are found over on Atlas Obscura. Describing the formation of the aragonite flower, Karin-Marijke Vis writes:
A helictite is a distorted form of stalactite. The cave formation changes its axis during its growth, defying gravity by the force of crystallization of the mineral, often growing in curving or angular forms (contrary to a stalactite, which hangs like an icicle from the roof, or a stalagmite, which rises as a column from the floor). It is still a mystery as to how they are generated, although there are several hypotheses (such as the formation of capillary forces, impurities in the deposited calcite, and/or air currents).
The aragonite flower (shown above) is unique in the world. The world's second largest of the rare speleothem grows in the Caverna da Torrinha. The formation of calcium carbonate resembles a flower, hence its name. This cave features a helictite with an aragonite flower on its tip, which has become the symbol of this cave, as well as of all other caverns in the region.
Read more, and see more incredible photographs, on Atlas Obscura