Falkenhagen is c. 17 km NW of Frankfurt on the Oder (on the Polish border.) It is an extremely large bunker north east of the village, construction commenced in1938. The area remained closed from 1938 - 45. The castle at Falkenhagen was demolished in the process. A standard gauge rail link (initially narrow gauge) was laid in with rail tracks running right through the bunker at one of the higher levels.  The open tunnel entrance (on the eastern side) is disguised inside a camouflage-painted corrugated iron shed. The interior of this large rail tunnel is now much altered by subsequent internal building changes and additions including the installation of NBC proof doors, tiled decontamination rooms and the like.
 

An extension from the main bunker appears to have been used for the storage of chlorine tri-fluoride ClF3, an exceptionally unpleasant compound, which was manufactured from 1938 and during WWII in surface plant,
presumably the gas was stored under pressure in liquid form (the liquid boils at c.12º C at normal air pressure.)

What the Germans proposed to do with this extra-ordinary material is far from clear.  Was it to find use as an explosive, a rocket fuel, an incendiary, or for chemical warfare?  Possibly incendiary uses are the most
likely, in view of the expense of manufacturing, storing, conveying, and delivering the compound precisely and in quantity.  It would presumably have been stored and conveyed under pressure in liquified form (as is chlorine), and perhaps nickel cylinders were used as some such metals develop a tough 'passive' metallic fluoride layer which prevents further attack (the 1930s are too early for fluorine-proof PTFE (poly-tetra-fluoro-ethylene) cylinder linings I think.) Our guide mentioned that chlorine tri-fluoride was 'tried out' on the Maginot line - thus supporting the view that it was to be used directly for offensive purposes.  Introduction of the gas (or rapidly /
explosively evaporating liquid) would result in exposed concrete and steelwork of air intake and filtering systems being seriously corroded . but how such material was to be delivered with precision to the intakes is problematic?  On the other hand, perhaps the chlorine tri-fluoride was intended only as an intermediate in the manufacture of, for example, SARIN?

Fluorine for the manufacturing process was generated on-site by electrolysis of a molten metallic fluoride (perhaps potassium fluoride KF?), derived from the common mineral fluorspar (CaF2) imported from Bavaria.

The storage area was on the lowest floor of the bunker extension, in 64 small rooms (four lines of 16) .  we were told the containers (full) weighed 2.5 tonnes, containing 1 tonne of chlorine tri-fluoride each and that there
was provision for 'drowning' the facility in massive volumes of water in the event of problems (? perhaps explosive instability) There are (surprisingly) small apertures at one edge of the ceiling in each room to provide for water entry / fumes escape and presumably the rooms have not been modified since first built . They have normal width doors (and provision for pressure-tight sealed doors?)  At a higher level, above the storage chambers, pressure relief valves remain.  On the ground surface above there is a high-capacity water tower, and one or two fumes escape shafts.  I saw no evidence of explosion or corrosion damage to the building fabric, so presumably the chlorine tri-fluoride remained under full control at all times. An overhead 'monorail' system was used for moving the containers around on the lowest floor.

There were altogether five surface chemical plants at this site, although we did not visit any other than 'domestic' surface buildings.  The world's first industrial-scale fluorine production commenced here (we were told),
and all data went to the USA (with von Braun?) Quite how the Americans got all this from the Russian-occupied zone was not made clear.  Other factories at surface made Tabun (from 1938), Sarin ('six times more
effective', from 1944), etc.  The Sarin factory was about 70 - 80% completed when the Russians arrived.

Grinding mills (to process fluorite raw materials) were in buildings later used by the Russians for saunas and showers. The chlorine tri-fluoride plant occupied four surface buildings  There was a manufacturing capacity of 500 tonnes per month . The establishment was known, in WWII, as 'Institute East.'  The chlorine tri-fluoride was codenamed N-stoff? Everything of importance was removed to Bavaria as the Russians advanced in this area.

The main bunker is on four floors. The Russians arrived here in April 1943 (the Germans had abandoned the bunker in February 1943) but decided they had no use for the structure from 1945 to 1960, and it closed temporarily in 1961. It was used February - April 1945 as a Russian hospital. The Russians subsequently created the standard gauge rail tunnel into the bunker. It is 180 m long and was intended to be used as a command post for the Berlin area.

The NBC proof door weighs seven tonnes. The bunker has 400+ rooms and it is 'very easy to get lost!'  Personnel 350 - 400 men? Cables reached the bunker via the Baltic seabed (bypassing Poland) but all Russian cables have been cut and they do not appear in East German records! Officers' rooms had wallpaper!  The Bunker is stripped of equipment; the Russians took from November 1990 to October 1992 to remove the equipment, and then left.

The third-level extension (westwards) was built by the Germans. The 60 m long upper levels used by the Russians (after chemical production ceased) as dormitories there were 64 'bedrooms' (cf. Storage chambers in lowest level). There is a large abandoned power station (not examined) on the surface with six diesel generators, these have now been removed, and the Russians used this space (seen) as a gymnasium . Amongst the surface buildings are barrack blocks (mural decorations in domestic rooms), a social centre (mural), etc. Characteristic Russian brickwork very prominent, white bricks, with 'far to many' long vertical joints!