Considerable amount of witnesses are found in the heavens and the earth for the facts of the Bible. Whereas here, the witnesses that testify, are the depths of the earth. 
Is there anything, which does not testify that the Bible is indeed a factual truth? This book speaks about the biblical cities, events, people, and other valuables that are derived from archaeologically excavated evidences. 
When we read the Bible, based on the historical background, it becomes evident that Bible is not a fiction but a fact.