Dakhma is a cilindrical construction used by Zoroastrians a type of sky burial, for exposure of the dead to scavenging birds for the purposes of excarnation.
We read some interesting details by Fosco Lucarelli:
Towers of Silence: Zoroastrian Architectures for the Ritual of Death
Zoroastrianism traditionally conceives death as a temporary triumph of evil over good: rushing into the body, the corpse demon contaminates everything it comes in contact with. The flesh of a dead body being so unclean it can pollute everything, a set of rules had to be created in order to dispose of the corpse as safely as possible: as the natural elements of earth, air and water are sacred, the corpses were not to be thrown upon the water or interred. Cremation was also forbidden, as fire is the direct -purest- emanation of the divinity. Hence a complex ritual was developed, in which the corpses would be eventually exposed to birds of prey and thus devoured, in a final act of charity. After death every division of class and wealth disappeared, for all deceased would be treated equally.
A proper architectural typology was invented solely for the purpose of burial’s ritual: transported in the desert by nasellars (traditional zoroastrian pallbearers), the bodies of the deceased were then carted onto sandstone, forbidding hills, to be eventually disposed on cilindrical constructions called Towers of Silence. A Tower of Silence, or Dakhmeh, is a structure laying on the top of a hill, consisting of concentric slabs surrounding a central pit. The bodies were arranged onto four concentric rings: men, outermost, than women and children. Despite the fact the the birds of prey needed less than an hour to leave nothing but bones, the remains of the dead were left bleaching on the upper circles no less than a year before the nasellars could come and push the skeletons onto the underlying ossuary pit. Running through sand and coal filters, the disintegrated bones were eventually washed away in the sea.
A guardian traditionnaly lived near the Tower of Silence, and was the sole person allowed to handle the ceremonial procedures, while relatives of the deceased stayed in a house below, and were forbidden to enter.
Iranian Zoroastrian discontinued this ceremony, and the Dakhmeh were banned in the 70′s; conversely, Parsi modern-day Zoroastrians in Mumbai and Karachi still mantains the tradition of burial by exposure, through the use of their own Towers of Silence.