What is the part of the iceberg that is not the tip called?

Erm... since 90% of the iceberg is underwater, that basically is the iceberg. What's the part of your body called that's not your head? It's just your body (with or without the head).

But note the usage in this Wikipedia article which specifically refers to the "underside" of a berg...

Seabed gouging by ice is a process that occurs when floating ice features (typically icebergs and sea ice ridges) drift into shallower areas and their keel comes into contact with the seabed.

Per my comment to another answer, bummock seems "less correct", as the hummock/bummock distinction is specifically associated with ice masses re-formed from pack ice (frozen sea water). Whereas to my mind, an iceberg implies frozen fresh water (bits broken off glaciers, or ice-shelves primarily formed from falling snow).

"Tip of the iceberg" doesn't appear to be in anyway a scientific term, and is just used idiomatically.

However, the word "iceberg" comes from the Dutch word, ijsberg, meaning ice mountain. Therefore, using mountain terminology may be appropriate here. My personal preference would be the "base of the iceberg".

If that makes it sound more like the very bottom of the iceberg, you may be better off going simply with the "underwater portion". This is the preferred terminology for encylopediae, but doesn't sound as good for a metaphor.

The underwater portion has no specific name that I can find. I would call it the bulk of the iceberg. I would also feel free to call it the submarinal portion, if needing a name for it.

Since iceberg is an ice mountain, compare: what is the part of the mountain called which is not the peak? If there is such a term (I can't find that, either), I'd say you have your answer. There are many parts of a mountain, including the peak, crest, base, slope, face, and more.

Bummock is the bottom part of the berg and Hummock is the top part.

Here is a good link I found that explains better. I know I saw something on National Geographic channel a few years ago with the names and drought (sp?) measures the bummock.

Now FumbleFingers makes a few points about bummock. Well he is right, kind of. It has various definitions. I don't disagree that one of them is "broken ice under the hummock, forced downward by pressure". What he is describing is a description for an iceberg. Also bummock seems the industry standard (oceanography) to describe the submerged part of froze ice which the user was asking for. As noted in my link above and Susan's link it means the bottom of an iceberg.

As for the use of keel I find it was lazily used in a couple articles. It refers to the bottom of a "boat". In the same articles they referred to the top as a "sail". I think it was more for analogy purposes than giving it an actually name. For sure joe-average-reader wouldn't understand the bummock usage. So I won't say that keel is wrong but it's just a descriptor not the word. We can use the sail/keel analogy for anything floating in any body of water and there is no specificity to icebergs.

My internet research was similar there appears to be no concise term that describes the part of the iceberg under the surface. Based on my personal experience leads me to make a choice of word becomes my personal preference for the writing task at hand. For me this involves considering my readers, my context and my writing style. My choice was "base" for my purpose and preference it works well with the word "tip".

Based on my engineering experience the two terms "bummock" & "hummock" could be a type of "technical jargon". It's nice to know that such jargon exists for subject matter experts that study snow, ice and glaciers for fun or profit. I find these two term would confuse my readers for the purpose of what I writing or require me to write longer to explain why what the are and would dilute the message I am trying to communicate.

Icebergs in the north (Atlantic ocean) have irregular shapes like mountains. Icebergs in south (Antarctic ocean) have more regular shapes like blocks (squares, rectangles, etc.) with the surfaces being quite flat. So, the "tip" of an iceberg refers to those we find in the north (Atlantic ocean). Of course, the icebergs of Antaractica were discovered long long after the discovery of the northern icebergs and so the idiom.

The thing is, the term "the tip of the iceberg" is an idiom with a specific metaphoric meaning: It means the visible and apparently innocuous portion of some much larger (and generally much more threatening) entity.

As such, somehow saying "the rest of the iceberg", even if it can be said with an "official term" (let's say "untip"), does not really imply a metaphoric meaning and is doomed to be unsatisfying.