the great fire of Copenhagen in October 1728

yellow on map 69 streets burned and 3650 families were made homeless

the Copenhagen University library, mss and archives
the town hall and its archives burned

astronomical instruments and great globe made by Tycho Brahe

more
http://en.wikipedia.org/wiki/Copenhagen_Fire_of_1728
the cultural loss is still felt today.

The University of Copenhagen library was without a doubt the greatest and the most frequently mentioned of such. 35,000 texts and a large archive of historical documents disappeared in the flames.^[35] Original works from the historians Hans Svaning, Anders Sørensen Vedel, Niels Krag, and Arild Huitfeldt and the scientists Ole Worm, Ole Rømer, Tycho Brahe and the brothers Hans and Caspar Bartholin were lost.

Atlas Danicus by Hansen Resens and the archive of Zealand Diocese went up in flames as well. The archive of the diocese had been moved to the university library the very same day the fire started.

Several other book collections were lost as well. Professor Mathias Anchersen made the mistake of bringing his possessions to safety in Trinitatis Church.

Árni Magnússon lost all his books, notes and records, but did manage to rescue his valuable collection of handwritten Icelandic manuscripts

At Borchs Kollegium 3,150 volumes burned along with its Museum Rarirorum containing collections of zoological and botanical oddities.^[36]

The burned out observatory in Rundetårn had contained instruments and records by Tycho Brahe and Ole Rømer.

The professors Horrebow, Steenbuch and the two Bartholins lost practically everything. And on top of all that

a large part of the city archive of records burnt along with city hall.

and from http://www.udstillinger.dnlb.dk/astroweb/Observatorier/