There are a few things, mostly advents of the ancient or classical periods, that we cannot replicate – not because it wouldn’t be useful or nobody wants it, but simply because the knowledge required to do so has been lost. Greek Fire is a great example of this. A substance that can be stored and transported safely in a ship, then somehow lit aflame and projected (supposedly to almost 100 feet away) at an enemy vessel. Granted, this would be somewhat useless in ship-to-ship warfare now, but there are now much more practical applications for this. On the military front, combat flamethrowers often suffer from 1 major weakness – the chemicals are incredibly vulnerable to ignition by enemy gunfire. One account of a great Greek sea battle (against the Persians, I believe) tells how the Greek ship carrying Greek Fire was directly struck by lightning, killing half the crew on board, and yet the Greek Fire in the hold did not detonate, and the remaining crew pretended to be dead, luring the Persians into boarding range before turning the Greek Fire upon them, roasting them alive at point blank range. The point here is that it would be a stable, liquid fuel with detonation properties similar to modern C4, but the inherent transport benefits of a liquid. To this day, nobody knows the formula to make it, and centuries of chemistry have failed to even come close. Of course, a much more controlled burn could have use in construction and even medical fields. Heat Shielding for aircraft and spacecraft is tempered with incredibly expensive, highly purified fuel due to the disaster that would result in fuel residue being left on the shield. Burn victims are, strange as it sounds, sometimes burned a little more to create enough burned tissue for other treatments to take hold, yet common fuels such as Butane have inherent medical hazards, and their storage in a hospital setting increases fire hazards in an already highly explosive environment. If it could be controlled with the same precision as more volatile fuels, Greek Fire might solve the problems in these fields and more inherent to virtually all other liquid fuel.

That said, I just want to say, the number of “tools” that are no longer in use can probably be counted on your own ten fingers, so the point is still pretty much valid, and I’m willing to bet that out of 7 billion people, there’s at least one who knows SOME of this, and just doesn’t realize what they’ve accomplished or isn’t willing to share.