What is a safe way to descend down a hole of unknown depth? I found a seemingly deep hole in the woods that I want to descend down into and explore.

I was thinking of getting two regular ropes, tying each one to a separate tree and then making a firm tie around my waist with one of them, and just holding onto the other one (just as an extra safety measure) as I repel down. In case the first rope breaks apart or somehow the knot is untied, I still have a backup.

I will of couse carry my cellphone with me

>pic related
dropping a small rock there, I counted 3 full seconds before it hit something. You can't even see the bottom when shining with a flashlight