Which Technology?

I was thinking of something I've heard many, many times. I couldn't find a thread on this so I figured I'd start one. Many a time that I had watched a show on medieval times, I remember hearing how today, we cannot master the art of sword making. With technology as advanced as ours, we cannot manage to discover how these blacksmiths made a sword? I'm not sure if that is true or not, but let's say we have people, or aliens, studying our people hundreds or thousands of years from now. I was thinking... what technology of ours would THEY not be able to replicate? I couldn't really think of an answer. I know there are many things on earth that ancients built that we have no clue really how to. So my question to all of you is, What technology will future people not be able to replicate, that we use today?

Since I cannot think of anything right now, I'll just say a Microwave since that's, for some reason, the first thing that popped in my head. I'll reply back sometime when I think I've come up with something.

How bout the technology involved in creating a major religion? Imagine all the writters, makers of weapons, those which can stage look-a-like miracales, etc...
Today this is done with the technology of tv in which midless celebs go in front of tv and state their claim for scientology, kabal or what not.

well human civilization lasts that long then they will be able to replicate everything because we will have permanent records of this data(atleast alot more durable), compared to those times when an art like sword making was probably passed on by mouth.

Knowledge.

If something catastrophic happened, we'd lose just about everything because we no longer store knowledge in written words on stone but in highly complex binary data that you can only read with the right equipment.

This of course would take a major catastrophy and at least a generation without the infrastruture to build digital tech.

I'd never thought of that. That would probably be number 1 on the list.