What is it that they're kicking?

It appears to be a request image, about some sort of Remilia football...

kurokoori said:
輝夜ＶＳ妹紅 ～超優雅な蹴鞠遊び！～

輝夜「さっさとボール落として楽になりなさい、妹紅！」　　妹紅「ふざけんな！地面に落としたら半径１０ｍが爆風で吹き飛ぶようなものてめぇにくれてやるよ！」　　　　　　鈴仙「師匠、なんであれだけ蹴り続けてるのにあれ爆破しないんですか？」　　永琳「月の科学力のおかげ。」　　　　　慧音「・・・・・・・・・」　　■前にリクエストで描いた蹴鞠遊び【illust/13308709】をちょっとだけ台無しにしたような感じの絵。殺し合いの仲だから仕方ないと思う。

Kaguya VS Mokou ~Super Elegant Kickball!~

Kaguya: "Just let the ball drop already, Mokou!"
Mokou: "Fuck you! If this ball hits the ground, the explosion'll take out everything within 10 meters!"
Reisen: "Master, how come it doesn't just explode from them kicking it?"
Eirin: "That's the moon's scientific prowess at work."
Keine: "........."

Kind of feels like I've ruined the spirit of the previous request image (illust/1330879). They're killer friends, so what can you do.

Take note,Mokou's hands are still in her pockets. She hasn't even gotten serious!

Tetrominon said:
Kaguya VS Mokou ~Super Elegant Kickball!~
Kaguya: "Just let the ball drop already, Mokou!"
Mokou: "Fuck you! If this ball hits the ground, the explosion'll take out everything within 10 meters!"
Reisen: "Master, how come it doesn't just explode from them kicking it?"
Eirin: "That's the moon's scientific prowess at work."
Keine: "........."

So It's basically touhou soccer?

AHHH ATATATATATATATATATATATATATATATATATATATATATATATATATATATATA ECT.