These conical, seven-century-old caves in Kandovan, Iran are made of ancient volcanic ash. They're still occupied and open to tourists visiting northwestern Iran.
From the Zoroastrianism Heritage Institute:
What makes Kandovan village so unique is that many of its homes have been made in caves located in cone-shaped, naturally formed compressed volcanic ash formations that make the landscape look like a gigantic termite colony. [...] It is our understanding that the unusual cone formations were formed from volcanic ash and debris spewed during an eruption of Mount Sahand being hardened and shaped by the elements over thousands of years. The formation of volcanic ash cones is local to Kandovan. Elsewhere, the ash blanketed the land. The existence of a high volume of ash and pumice far from Sahand's crater indicates that Mount Sahand erupted with a gigantic explosion in the distant past. Sahand's rock is about a million years old and the last eruption of Mount Sahand is thought to have occurred within the Holocene epoch, that is within the last 11,000 years.
June 2005, image via Fabienkhan at WikiMedia.