The Indus Valley civilisation (3,300–1,900 BCE), located in Northwestern Indian subcontinent, is sometimes suggested to have been Dravidian. Already in 1924, when announcing the discovery of the IVC, John Marshall stated that (one of) the language(s) may have been Dravidic. Cultural and linguistic similarities have been cited by researchers Henry Heras, Kamil Zvelebil, Asko Parpola and Iravatham Mahadevan as being strong evidence for a proto-Dravidian origin of the ancient Indus Valley civilisation. The discovery in Tamil Nadu of a late Neolithic (early 2nd millennium BCE, i.e. post-dating Harappan decline) stone celt allegedly marked with Indus signs has been considered by some to be significant for the Dravidian identification.