The Roman scholar Pliny the Elder described underwater concrete structures that become "a single stone mass, impregnable to the waves and every day stronger."

The Roman scholar Pliny the Elder described underwater concrete structures that become "a single stone mass, impregnable to the waves and every day stronger." https://www.nature.com/news/seawater-is-the-secret-to-long-lasting-roman-concrete-1.22231 https://www.nature.com/news/seawater-is-the-secret-to-long-l... Sounds like they knew.