  Here we report the first evidence of the inverse energy cascade in a flow
dominated by 3D motions. Experiments are performed in thick fluid layers where
turbulence is driven electromagnetically. It is shown that if the free surface
of the layer is not perturbed, the top part of the layer behaves as quasi-2D
and supports the inverse energy cascade regardless of the layer thickness. Well
below the surface the cascade survives even in the presence of strong 3D eddies
developing when the layer depth exceeds half the forcing scale. In a bounded
flow at low bottom dissipation, the inverse energy cascade leads to the
generation of a spectral condensate below the free surface. Such coherent flow
can destroy 3D eddies in the bulk of the layer and enforce the flow planarity
over the entire layer thickness.
