Last summer I noticed water coming up from my basement, always in the same spot. The rest of the basement is bone dry and we have french drains so I was baffled. This winter, I tore up the tile that was in that area, and lo and behold, I found a hole roughly the size of a golf ball. I am not able to confirm whether this is where the water is coming from because nothing is leaking in winter, but this is the exact spot water always pooled from, so I assume it is groundwater coming up.
We are recent owners of the house, so I have no idea how the hole got there. I want to fill it. Someone warned me about water pressure building up if I did that, but there is a large sump hole very close to this.
Here are some pictures with my finger for scale:
How do I fill this?