A black cave deep under the earth in minecraft?

I'm new to Minecraft, and I decided to go on Creative and dig as deeply as I could. So I chose a spot near a mountain and started digging. It wasn't very long until I found myself in a deep underground cavern filled with what appeared to be glowing eyes or lights. I flew out of there as fast as my space bar would let me, but I still don't know what it is. Can anyone help me? Thank you.
Added (1). Just so you know, I am terrified of the dark, both online and in real life. Any tactics for a scardey-cat trying to be brave enough to enter wherever this is on Creative?

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