Here is a standard Time Capsule in Inox with a Life Time of 200 years, with 1mm of thickness :
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Lets try something more time resistant with a titanium and concrete !
One the paper, technical concrete like Fiber Reinforced Concrete could last 1000 years.
The next step will be to add a time resistant radio pulse emitter powered by an energy harvesting device.
watching! very interested to see this de-moulded. How are you going to control bubbles?