In this pre-1998 photograph we see a smaller tower and ancillary structure well to the south of a newer and larger scaled structure built to house the firefighting truck (although for all practical purposes the truck is always left outside on duty.)
In April 2000, we still see the old air-traffic tower, south of the fire station and even south of the entrance between corridors Four and Five, flanked by large trees.
Three months later, in July 2000, temporary fencing related to the renovation has been reduced significantly, revealing a drive that skirts the front of the building from north to south.
But a year later, in the aerial image taken by Maxho on August 25, 2001, we see a new, larger tower built north of and contiguous to the fire station. The old site can be seen as infilled with darker asphalt, which also fills in a grassy patch beside the fire house. In this way, I believe, an attempt has been made to disguise the new construction as resembling the old. To attribute a motive for the change, I would have to say the old tower was relocated out of the way of an impending strike zone.
All of these images were downloaded off Russell Pickering's site. That the area underground was the locus of strange and powerful weaponry is a dawning realization for some. In many images, a semi-circular line can be seen in the lawn directly in front of the strike zone. In reality, this was a depressed area which filled up with water during the fire fight. Russell Pickering wrongly characterizes the line apparently as tire tracks.
But in the uncropped and higher resolution image that Pickering posted dated August 25, 2001, available at www.maxho.com, the line can be clearly made out as a deeper, well-watered green.
And in this high-resolution image of the aftermath of the attack, not only the line but some apparent function can be determined. It seems as if hoses are draining water either toward or away from this installation. And the blue-tarp tent which was located front and center close by mid day of September 11th, may have a relationship or connection to this underground actively.
The Pentagon's mechanical plant, even after complete reconstruction as part of the renovations, still goes by the antiquated name of Heating & Refrigeration Plant, or in other words---H&RP, or HAARP, and quite the installation it appears to be too. Perhaps it has a weaponry function in addition to a climatology one. More shall be revealed!