For decades Afghan refugees have been sneaking into Iranian territory across the long, barren border, escaping the poverty and war in their own country.
The first waves of immigrants were Afghanistan's ethnic Hazaras, who share a common Shiite Muslim faith with Iran, and Tajiks, who speak almost the same language as Iranians.
But increasingly Afghanistan's ethnic Pashtuns, Sunnis who speak their own language, are also coming across the border and settling in Iran, anecdotal evidence and voices heard on the street suggest.
Life for them in Iran is not easy. Not only do they struggle with the language, but they can't open bank accounts or send their children to school.
After working hard for a couple of years, they gather their savings and go back to their war-torn country, often getting travel papers from the Afghan Embassy in Tehran or consulate in the eastern city of Mashhad.
Then, they come back, again and again.