You could build a megalithic concrete structure like the Hoover Dam. https://www.quora.com/If-humans-vanished-from-the-earth-how-long-is-it-estimated-that-the-

You could build a megalithic concrete structure like the Hoover Dam. https://www.quora.com/If-humans-vanished-from-the-earth-how-long-is-it-estimated-that-the-Hoover-Dam-will-hold-up https://www.quora.com/If-humans-vanished-from-the-earth-how-...