دخمه - dakhmeh (tower of silence) near yazd
Zoroastrians believe in keeping four elements clean in Nature: water, air, earth and light. So, they are not going to contaminate them. Instead, they keep them pure and protect them against impurities. The decomposing corpse is considered most pollutant and should not contaminate the elements.
This is why they leave the dead for the vultures atop a tower constructed above hills, called dakhmeh (place of the dead … towers of silence = as translated into english in 19th c.).