All together, 急にウサギの穴にとび込んだ = (it) suddenly jumped into a rabbit hole.
Full sentence: ウサギは早く走って、急にウサギの穴にとび込んだ = In relation to the rabbit (it) ran quickly and suddenly jumped into a rabbit-hole.
アリスもウサギの穴にとび込んだ = Alice also jumped into the rabbit hole.
The も particle
も (particle) is a non-logical topic-marking (“flagging”) particle like は.
It’s the only other non-logical topic-marking particle.
も is more specifically a topic-changing particle. It cannot be used unless there is already a different topic in the conversation that can be changed from.
アリスも
The topic up until now has been the rabbit, now the も particle is changing that topic to Alice.
も does not draw a distinction between topics like は does, when using も one is saying that the comment about this topic is the same about the comment about the previous topic. Sort of like an “In relation to x they also…”, with x being the も flagged topic.
穴の中はたて穴だった。アリスはすぐ下落ちた。
中 (なか) means “the inside”.
穴 (あな) means hole.
穴の中 (あなのなか) means “the inside of a hole”. More exactly, “holes inside”.
たて/縦 = vertical.
たて穴/縦穴 = vertical hole.
穴の中は竪穴だった = “The holes inside was vertical” (it means it went straight down).
「アリスはすぐ下落ちた。」
下/した = down/below.
すぐ = Direct.
すぐ下 = Straight down/directly down.
「でも驚いたことにゆっくりゆっくり落ちた」 = “But surprisingly she fell very very slowly”.
驚く/おどろく = surprised.
こと = thing (abstract).
驚いたこと = surprising thing.
ことに is the usage of に to turn a noun into an adverb (surprisingly).
“驚いたことに落ちた.” = Surprisingly fell.
ゆっくり adverb
First two kinds of adverb are く-stem adverbs (of adjectives), and に adverbs (for nouns).