problematic
(also problematical) adj
1 difficult to deal with or to understand 难处理的; 难了解的; 成问题的; 疑难的.
2 (esp of a result) that cannot be foreseen; doubtful or questionable (尤指结果)难以预料的; 有疑问的; 令人不解的.
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.