Have the website engraved on stainless steel plates. There are many affordable online services out there that would do that for you. If your descendants store

Have the website engraved on stainless steel plates. There are many affordable online services out there that would do that for you. If your descendants store the plates in a dry attic, they could last 500 years just fine. But… did anybody ever find something on their attic that was there for 500 years. That is very rare. I would assume that your great-grandchildren will throw them away because they need the space.