A subspace funnel, or warp funnel, was a theoretical phenomenon capable of connecting two points through subspace. The idea was proposed by Enterprise chief engineer Geordi La Forge in 2369, in an attempt to explain the disappearance of the USS Hera, a Starfleet vessel of which his mother was captain.
According to La Forge's hypothesis, the Hera picked up some residual subspace disturbance traces from the atmosphere of the gas giant Marijne VII whilst passing by the planet. Ten days later, 300 light years away, experimentation on the Hera's trionic initiators resulted in the formation of a subspace funnel, which was directed back to Marijne VII due to the disturbance traces. Geordi planned to use the USS Raman, a Starfleet vessel which had become trapped in the atmosphere, to initiate an inverse warp cascade, which he believed would reverse the effects of the funnel. It was subsequently determined, however, that the Hera had not been pulled into a subspace funnel, and the Hera was not recovered. (TNG: "Interface")