Archaeological exploration done in the 20th century suggests that the geographical area of Afghanistan has been closely connected by culture and trade with its neighbors to the east, west, and north. Artifacts typical of the Paleolithic, Mesolithic, Neolithic, Bronze, and Iron Ages have been found in Afghanistan. Urban civilization is believed to have begun as early as 3000 BCE, and the early city of Mundigak (near Kandahar in the south of the country) was a center of the Helmand culture. More recent findings established that the Indus Valley Civilization stretched up towards modern-day Afghanistan, making the ancient civilization today part of Pakistan, Afghanistan, and India. In more detail, it extended from what today is northwest Pakistan to northwest India and northeast Afghanistan. An Indus Valley site has been found on the Oxus River at Shortugai in northern Afghanistan. There are several smaller IVC colonies to be found in Afghanistan as well. An Indus Valley site has been found on the Oxus River at Shortugai in northern Afghanistan, shows Afghanistan to have been a part of Indus Valley Civilization. After 2000 BCE, successive waves of semi-nomadic people from Central Asia began moving south into Afghanistan; among them were many Indo-European-speaking Indo-Iranians. These tribes later migrated further into South Asia, Western Asia, and toward Europe via the area north of the Caspian Sea. The region at the time was referred to as Ariana. By the middle of the 6th century BCE, the Achaemenids overthrew the Medes and incorporated Arachosia, Aria, and Bactria within its eastern boundaries. An inscription on the tombstone of Darius I of Persia mentions the Kabul Valley in a list of the 29 countries that he had conquered. The region of Arachosia, around Kandahar in modern-day southern Afghanistan, used to be primarily Zoroastrian and played a key role in the transfer of the Avesta to Persia and is thus considered by some to be the "second homeland of Zoroastrianism". Alexander the Great and his Macedonian forces arrived in Afghanistan in 330 BCE after defeating Darius III of Persia a year earlier in the Battle of Gaugamela. Following Alexander's brief occupation, the successor state of the Seleucid Empire controlled the region until 305 BCE when they gave much of it to the Maurya Empire as part of an alliance treaty. The Mauryans controlled the area south of the Hindu Kush until they were overthrown in about 185 BCE. Their decline