Amber Room

In 1974, the Arecibo Observatory was used to send the first known intentional radio message transmission into space

01The 1974 Arecibo Message was aimed at globular cluster M13 — 25,000 light-years away, arrival date roughly 26,974 AD.
02Stephen Hawking publicly warned that contacting aliens could be analogous to Native Americans meeting Columbus — and we know how that ended.
03METI International transmitted 'Sonar' toward GJ 273, a nearby star, in 2017 without any international consensus or oversight body approving it.

A thermite mixture using iron(III) oxide

Nazis stripped a room of six tons of amber panels in 1941; eighty years of excavations, tunnels, and deathbed confessions have recovered nothing. Where do you hide a room?