然而，许多目击事件的确切原因仍然是个谜。
However, the exact cause of many sightings still remains a mystery.
然而，使用诸如此类的非实验数据来证明原因是非常困难的。
However, it is very hard to demonstrate causes using non-experimental data such as this.
然而，还有其他对生命是否存在于其他地方感兴趣的原因。
However, there are other reasons for being interested in whether life exists elsewhere.
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