Some believe from a passage of scripture in the book of Jeremiah 4:
23 I beheld the earth, and, lo,
it was without form, and void; and the heavens, and they
had no light.
24 I beheld the mountains, and, lo, they trembled, and all the hills moved lightly.
25 I beheld, and, lo,
there was no man, and all the birds of the heavens were fled.
26 I beheld, and, lo, the fruitful place
was a wilderness, and all the cities thereof were broken down at the presence of the LORD,
and by his fierce anger.
27 For thus hath the LORD said, The whole land shall be desolate; yet will I not make a full end.
28 For this shall the earth mourn, and the heavens above be black: because I have spoken
it, I have purposed
it, and will not repent, neither will I turn back from it.
this to them mean, there was a creation before Adam and that creation was destroyed by God. They believe God destroyed it with water before Noah's flood, but left the water to ensure everything completely died since Genesis 1:2 says the Spirit of God hovered over the face of the deep (water). They believe that Genesis 1:1 literally meant there was a creation of everything and it was finished as it was good to God. Then Genesis verse 2 reveals a different view of this finished/completed world in chaos, complete darkness, and under water.
If this is actually true, the world could be hundreds of thousand years old. I don't buy the billions of years old, but I can definitely see this world being older than 6,000 years. For one, it's believed that Cain killed Abel when he was in his 20's and then after he left found more people including a wife. If this is true, and we know Abel and Cain were Adam's first children born, it would have been impossible for Adam to populate the earth already for Cain to find himself a wife. This basically proves other people were here when Adam was created, or were also created not long after but never mentioned until Cain found a wife.
I definitely do not buy the world being 6,000 years old. We can look at ice rings, tree rings, rock definitions to see some of them are well over 20,000 years old. But I love mathematics and science because they both ultimately prove the existence of God!