To answer the first part of your question, I'd like to say that our studies were very extensive.
你問題的第一部分,我要說的就是我們的研究非常廣泛。
Apparently what happened was that water collected on these tar pits, and the bigger animals like the elephants ventured out on to the apparently firm surface to drink, and were promptly bogged in the tar.
瀝青坑里積存了水,大象那樣的大動物冒險到似乎堅固的水面上去飲水,立即掉進了瀝青坑。