Several hundreds of thousands of scrolls may have been collected during the 300 years the Ancient Library of Alexandria existed.

There is lost or forgotten knowledge either at the individual, population or humanity level

An example of forgotten knowledge is the knowledge of the meanings of words - in our own languages.

Several hundreds of thousands of scrolls may have been collected during the 300 years the Ancient Library of Alexandria existed.

Similar case is The great library of Baghdad.

Priceless knowledge from  the Aztecs and Inca was lost forever.

interesting ! There are surely numerous examplers of forgotten knowledges in Anatomy. I'll try and find some examples.

Quite interesting. 

Another example, dear Amir, is the Al-Hambra of al-Andalus!

How the pyramids were built.

Ancient farming practices?

Old sewing machines which were used to stitch fabric and other materials together with thread.

Here are some examples listed! Dear @Amir, we at Balkan very often forget the wars that are behind us, instead to learn from! 

http://www.quora.com/What-are-the-oldest-examples-of-forgotten-knowledge

http://www.quora.com/What-are-the-most-important-examples-of-lost-or-forgotten-knowledge

Is an ancient language forgotten knowledge when it repeats what is explained in another still existing language?

The Serbian Cyrillic alphabet is going to be forgotten also!  Given current trends, experts estimate that within a few generations, more than 50 percent of the estimated 7,000 languages spoken in the world today may disappear. The consequences of this

would be tragic.

http://en.wikipedia.org/wiki/Serbian_Cyrillic_alphabet

http://archive.unu.edu/globalization/2008/files/UNU-UNESCO_2008_Globalization_and_Languages.pdf

The Inca masonry of south America is probably the finest the world has ever seen.

Evidence for drilling in ancient Egypt.

Even today it would be extremely difficult to build Hagia Sophia.

Perhaps old forgotten manuscripts will be rediscovered in the future, e.g. music composers?

Dear All,

I think unlimited ideas and knowledge fragments have been lost because of the uninterested or hostile environment.

Dear All,

Many secrets of the construction of traditional Hungarian bow have been lost.

Dear All,

I am afraid that preparation of traditional and area specific food items will be lost because of spreading of commerce foods like hamburger and hot dog.

Dear All,

The mutual respect and politeness, even neat courting as base attitudes of human relationship are vanishing.

Dear All,

The biggest trouble is that many people try and is able to survive contradicting the truth and spreading that it is possible to make success without knowledge and efforts. This is the perfect denial of traditional knowledge.

This has been started a process to extinguish the knowledge sparks.

The secret to Roman concrete lies in its unique mineral formulation and production technique. As the researchers explain in a press release outlining their findings, “The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together.”

http://newscenter.lbl.gov/2013/06/04/roman-concrete/

Why preserving old knowledge in a dynamic world? Nostalgy or more than nostalgy?

Excellent question dear Marcel. It is definitively more than nostalgia.

Maybe mankind history is kind of repeating circles of knowledge. 

The problem it is not only with lost or forgotten knowledge, but also with false labels we usually put on previous philosophers and scientists. An example is G. Frege. All they know him as the grandfather of analytic philosophy. In her last book D. Macbeth, changes completely the picture of Frege! There are many such examples. For this historical research is necessary!  Foundations is derived by examining historical events in depth and by “concentrate the essence of practice and in turn use the result to guide practice”.  In this way the lost knowledge will be minimised!

Is knowledge lost/forgotten or hidden somewhere?

Costas thinks that lost or forgotten knowledge might differ from publicly accessible knowledge?

To be honest, I lost and forgot them, there are too much. ;-))

Could all individual life experiences from people from the past be defined as forgotten/lost knowledge?  

Does this imply that all biology-based mental knowledge from other people will be forgotten/lost, also accepting that this mental information is not necessarily reflected in publicly accessible constructions (buildings, books, compositions, paintings, objects....)? 

Example:

Are the billions of dream experiences all over the world lost knowledge? 

Dear Marcel,

You follow a nice strategy! You answer, with questions!

Yes dear Costas,

I LOVE and are accustomed to HIS QUESTION MARKS: YOU HAVE DO THE WORK AS READER; not the producer of answers. Not so bad technique.

I remembered!

Thanks to your comments and remarks, dear Marcel, I just remember what I had been looking for. Your comments always make me think and re-think, and as I was «upvoting» your remarks,  I thought that I don't enterely agree with you.

It is not only a question of nostalgia. I terms of scientific research, many great ideas fall into forgettness (if you may say this word), most often for the lack of methodological instruments to demonstrate. In this case, you have to wait a few centuries to demonstrate.

I wouldn't go so far as the lost scrolls of Alexandria.

The anatomical example I was looking for, comes from the 17th century and the long forgotten theories of Thebesius on the venous system of the heart.

http://drsocial.org/forums/topic/298/02-true-thebesian-veins/view/page/1

(You'll find his original text in the above link).

Even if he was absolutely right in his theories, that the cavities of the heart could drain directly to the myocardial veins, these theories failed to be demonstrated in his times, either for lack of interest - because the physiology and pathology of the heart was still poorly understood - or for lack of development of angiologic imaging techniques.

Over two centuries later,, with the development of new imaging techniques in my country (angiographic techniques), and with the evolution of the practical medical interest to study the heart and circulation, the (now) emeritus Professor Esperança Pina developped new techniques to study the anatomy of the circulatory system and demonstrated these veins with precision. 

This was the subject of his doctoral thesis in 1972. Even if his own researches were quite original, he was humble enough to reffer these veins as the Thebesian veins, in honour of the man who originally theorized on them.

I hope that I didn't bore you with my double-century long story, but I realised that it could answer a few of your previous remarks, here. And nothing like a practical example to illustrate our theories. 

Dear Maria,

Thank you ! 

Dear Amir, another important medical nearly forgotten whole person is the Portuguese humanist AMATO LUSITANUS, who spent some years of his life in Sarajevo.

Dear Maria,

Amato Lusitanus,who first discovered the Blood circulation phenomena, after staying for some months in Dubrovnik left the city for Thessaloniki, the Ottoman Empire. I wasn't aware of the fact that he spend some time in Sarajevo.

How much knowledge presented in unpublished PhD thesis chapters are forgotten, and perhaps lost or rediscovered?

Dear Marcel,

I think a lot of know how is hidden. But I have a well working idea. Try to publish.

Dear Hanno,

I am not personally concerned, but I think there are a lot of Master or PhD manuscripts of which less than half was published, also because students had to stop research. 

Several decades ago, the PhD manuscript had to be completely written in the local language, perhaps complicating publication in journals? How much of these old PhD manuscripts are still accessible or verified by new generations of teachers/ students/researchers?

Dear Marcel,

may be. But I remenber my old scientific situation as growing PHD. I was nearly forced to publish, I learned a lot, my supervisors developed to friends.

My urgent advice, publish

For 1500 years the old cemetery in the desert in the area of the Egyptian province of Faiyum, a hundred kilometers south of Cairo, were buried more than a million mummy, writes "Focus".

Live Science published the results of previous excavations which are in Egypt for three decades dealing with researchers from Brigham Young University in Utah.

Based on these data, it is now found about 1,700 mass buried mummies, but judging by the area of 300 hectares occupied by the cemetery, today known as Fag el-Gamous, by digging the density and the depth to which the bodies were buried - without luggage - archaeologists conclude that they has more than one million.

We can not avoid TESLA! The Lost Inventions of Nikola Tesla are given! Lost knowledge or forbidden knowledge!

http://altered-states.net/barry/tesla/

http://www.kerryr.net/pioneers/gallery/ns_tesla10.htm

http://www.ancient-world-mysteries.com/tesla.html

http://www.bibliotecapleyades.net/ciencia/energygrid_harmonic695/energygrig03.htm