Haunted ferryboat.

When an ancient redwood-20' diameter and 2000 years old-is cut down, a strange artifact is discovered embedded in it.  Rings allow it to be dated.  More found in more trees--always at the same timespan, according to rings.  Perhaps additional dateable finds in geographic strata unearthed in mines, or ancient ice floes.

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When they developed the Santa Clara valley, they cut down the orchards, but left the roots--and they remain as a tangled underground highway for termites--and maybe something worse.  And are there deeper, more ancient conduits?