I second Arethuza, if you count construction projects there are some impressive long term projects. Some, such as Chichen Itza (400yrs) or Stonehenge (1,600yrs)

I second Arethuza, if you count construction projects there are some impressive long term projects. Some, such as Chichen Itza (400yrs) or Stonehenge (1,600yrs), have direct scientific significance as well.