Like we learn from the opening words from Kabbalat Shabbat (Tehillim 95:1-4) we can understand the Upper Wisdom of the Torah via the allegorical models of the Lower Wisdom. The Upper Wisdom refers to the wisdom of the Kabbalah. To a large extent, what is found in Midrash is based upon these inner teachings of the Torah. The Lower Wisdoms refers to the 7 sciences (mathematics, chemistry, physics, mettalurgy, etc.).
What you are translating as "the middle drop of the waters of the universe crusted", is actually from the Hebrew:
גלדה טיפה האמצעית
The middle drop (of water) froze...
This root גלדה is better translated as freezing, becoming ice. What it is describing is the transition from liquid to solid. The liquid state is more like what is described in modern Physics as the plasma state of existence at the beginning of the Big Bang. The point of transition from Liquid to Solid is referred to as the point in the expansion of the universe when Large-Scale Structure Formation started to take place.
A very good allegory for the firmament (Rakia) in this case is the concept of an inversion layer as discussed in chemistry. What is described as Rakia would be the analog of the inversion layer.
In this case, the concept of the transition from dextro-rotary (right rotating particles) molecules to their chiral (mirror) counterparts, levi-rotary (left rotating particles) molecules corresponds to the mirror aspect of the transition from the spiritual to the physical.
A second good allegorical model from chemistry is called a phase separation or to be more precise, and Aqueous Bi-Phasic System. In this case, there are quite literally two Waters, aqueous solutions, which are separated or distinguished by the transitional phase which is also Water.
Again, the Rakia is the layer of transition (the phase separation between the upper waters and the lower waters). That transitional point also corresponds to the ideas of Nekuda, Kav and Shetach which are discussed in the Kabbalistic areas of the Torah discussing the same subject.
The significance of this aspect is that like is understood from the geometric definition, a point (Nekuda) and a line (Kav) do not occupy space, meaning the type of existence as we know it. Similarly, a plane (Shetach) occupies space only from the point of observation above or below the plane itself. If one observes the plane from the level of the plane itself, it has the appearance and qualities of the line (Kav) which means it does not occupy space. It does not appear to exist at all.
Another important concept that relates to this particular allegory is that Phase Separation is used for the process of performing extractions which corresponds to the idea of Birrur HaNetzutzot which we perform through the performance of the Mitzvot and the reciting of the accompanying Brachot.