Which Australian orchid genus includes species that flower entirely underground?

The Australian orchid genus that includes species flowering entirely underground is Rhizanthella.

Known as underground orchids, Rhizanthella species are leafless and spend most or all of their life cycle beneath the soil or leaf litter. Unlike ordinary green orchids, they lack chlorophyll and obtain nutrients through mycorrhizal fungi associated with nearby plants.

Their flowers grow in compact heads surrounded by overlapping bracts. In some species, the head remains completely below ground; in others, it may reach the surface or protrude by a few millimetres. This unusual habit makes the plants extremely difficult to find and study. The western underground orchid, Rhizanthella gardneri, was first discovered in Western Australia during ploughing in 1928.

Rhizanthella is not a single species but a genus containing several Australian species, including R. gardneri, R. johnstonii, R. slateri, R. omissa and R. speciosa. The genus name combines Greek roots associated with “root” and “flower.”