@atyh Yeah, it’s the rebar. Take a look at Hashima Island for how not to do it. If you use the right concrete mix and get the proper coverage on the reinforcement, modern structures can last a really long time. Sometimes people do get it right and in a thousand years folks will be talking about how well we built stuff. Survivor’s bias. It’s like no one thinks about building things to last or to be efficiently recycled. Worst of both worlds.