I have a hole in the concrete floor of my basement. It's about the size of a quarter and is deeper than a chopstick (I didn't try putting anything deeper in there). I'm including a picture below.
I've lived in this house for a few years now. According to the realtor (we bought an estate property) there was a major project to replace some sewer lines due to a large tree growing outside near the house. This might not be related to the patched concrete that @Ecnerwal mentions, but I figured I would add this info in case it's relevant.
Question 1: What the heck could be causing that hole?
Question 2: Is this something I should try fixing or have someone come take a look at? I'm generally game for fixing it myself, but wanted to see if you think there are bigger issues that could be causing this.
Question 2a: Any recommendations for what I should do to get started?