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

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.

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).

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.

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.