Rome is home to all sorts of monuments from the ancient times, and while they're large and complex, one of the things that continues to boggle many people's minds is how they're still standing today.
It turns out that ancient Romans used a very strong concrete blend that is even stronger than the concretes used to build structures today.
With the ancient Roman concrete recipe calling a mixture of limestone, water, volcanic ash, and supportive rocks, the result of is a solid block that produces stronger bonds thanks to the strätlingite crystals that form while it sets.
If that wasn't impressive enough, some of the ancient Romans' concrete structures are only getting stronger as time passes; namely those that are exposed to the seawater. This is because the porous concrete is introduced to minerals from the water, which in turn strengthen the overall concrete structure.
It seems the Romans were doing something right, and perhaps we have a lot to learn from them, even today.