Drilling: what's underneath my subfloor?

Question

I have a squeaky floor and I'm trying to use a kit to screw the subfloor to the joists. However, while drilling to find the joists I can go down less than an inch before the drill stops going down and instead makes a noise like I'm drilling into concrete. The room I'm in is at ground level but there is basement below it. Would there be any reason for a cement-like level under the subfloor? How can I know if it's safe to fix the squeaks?

Update: I'm fairly new to home repair. I just bought my first house a few months ago. Anyway I pulled out a vent and didn't see anything too out of the ordinary. It looks like there might be two subfloor layers, one particle board and one plywood. It's possible I easily went through the particle board and I'm not used to how hard the plywood is.

Answer 1

Sounds like there could be some self-leveling concrete somewhere under there.

Self-leveling concrete is typically used to create a flat and smooth surface with a compressive strength similar to or higher than that of traditional concrete prior to installing interior floor coverings. Self-leveling concrete has increased in popularity as the degree of flatness and smoothness required for floor covering products has increased, with vinyl goods becoming thinner and floor tiles becoming larger, for example

It's possible the concrete is only in the low spots of the floor, so might not be visible (or extremely thin) at the vent you checked.

Since there's a basement underneath, if you can get access to the floor joists from below that might be a better way to stop squeaks.