Punctuation troubles in fiction

Question

Started reading Sword Art Online, and since it's my first time reading novels in Japanese, the first paragraph is already making me ask questions on Stack :P

職人クラスの酔狂な一団がひと月がかりで測量したところ、基部フロアの直径はおよそ十キロメートル、世田谷区がすっぽり入ってしまうほどもあったという。

I get the basic idea of the sentence, but the grammar coupled with unusual punctuation trips me up. What's going on in the middle part between the two commas? I'm not sure how it works, given that there are no particles or anything attached to キロメートル and it doesn't seem like the topic-wa relates to the rest of the sentence in any way, only to the part within the commas.

Also, if somebody knows what もあった is supposed to be in this context, I'd be happy to know.

Answer 1

Your translation (in the comment) seems fine. The comma after 十キロメートル is used to rephrase or explain the word before it. In English, this use of commas is known as "parenthetical comma", but in Japanese, we often use a single comma.

You can replace this use of comma with actual parentheses like so:

基部フロアの直径はおよそ十キロメートル（世田谷区がすっぽり入ってしまうほど）もあったという。
They say the diameter of the base floor was 10km (which was enough to contain the entire Setagaya Ward).

Similar examples (note that there are only one comma in the Japanese versions):

• 私の友達、花子はスキーが上手だ。
  My friend, Hanako, is good at skiing.
• 世界初の人工衛星、スプートニク1号は1957年に打ち上げられた。
  The first artificial satellite, Sputnik 1, was launched in 1957.

For this も, see this and this.