"Coffee grounds" at base = ant/roach droppings. They didn't hollow the limb—water entering a crack did. Decay spreads down over years.
Tree = shell of bark around empty heartwood. Will fall eventually.
Hire certified arborist (GA Arborist Assn) for hazard assessment. Don't guess.

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Carpenter Ants in Oak Trees: Decay Is the Real Problem
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Carpenter Ants and Decay in a Mature Oak: What to Do
Carpenter Ants in a 150-Year-Old Live Oak: Not a Tree Threat
Young Oak Vertical Bark Cracks: Stress Injury, Consider Replanting
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Why Is My Mature Oak Tree Shedding Leaves and Acorns Excessively?
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Is a Hollow Tree a Dead Tree? Understanding Heartwood Decay and Structural Risk
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