School Project

I'm doing a report where you have to explain a phenomenon or technology in a sci-fi novel using physics concepts. I'm doing a book called Roadside Picnic where there are points on Earth with abnormally high levels of gravity, enough to crush a person. Now I hypothesize that there's a dense material underneath the ground which is affecting gravity. I came to this conclusion because areas on Earth with more mass can have higher gravity so I just sort of took that to the extreme. Is that even possible in theory? I already know more mass would increase gravity and I know the amount of force needed to crush a person (~26k N, I used the amount of force a femur could withstand as in the books the victims are just straight-up flattened). Not even going that far is, it possible for a point on a planet to be so dense it is just noticeably harder to move or something.