At a guess, it doesn't look like sponge, it looks more like a poundcake.
If it was a sponge recipe then you probably overmixed or over folded it.
Texture looks quite solid, not light.
Its best to follow a video recipe off youtube than a book recipe if you aren't familiar with proper proceedures.
That is a sponge cake that has been over mixed. When you overmix a cake batter you develop too much gluten. The result is the leavening gets trapped in the strong gluten network and it tunnels through the cake batter as it bakes. So you get these tunnels that are baked into the cake.
You can tell it’s tunneling because an air pocket that you accidentally trap in the batter will be just a single small little hole. Tunneling will expand throughout the cake like this.
This cake also baked at too high a temperature. Look at how thick and dark the crust is.
It doesn’t matter if its plain flour or not. If the gluten in the flour is over developed, the air bubbles whether from mechanical leavening or chemical leavening or a combination of both get trapped in the gluten network and tunnel through the cake batter as it bakes.