S-V Agreement Question

Hi,

I have a good understanding of S-V Agreement but sometimes I get the subject identification wrong.

For example....

Excavators at the Indus Valley site of Harappa in eastern Pakistan say the discovery of inscribed shards dating to circa 28002600 B.C. indicate their development of a Harappan writing system, the use of inscribed seals....

Verb: Indicate
Subject: Discovery -> Why is the subject Discovery? Why isn't it Excavators?

Thank you for the help!