No one knows for certain what happened, but it's a fair bet that it was not an accident.
没有人知道究竟发生了什么,但几乎可以肯定这并非意外。
If there had not been a hard layer of rock beneath the soil, they would have completed the job in a few hours.
如果不是因为土壤下面有一层坚硬的岩石,他们的营救工作仅用几个小时就可以完成了。