There are six tunnels under the National Theater. Two of them are older than the theater itself and were there even before the construction of the Provisional Theater (Prozatímní divadlo), which was the predecessor of the National Theater, began. They were originally used to transport goods from riverboats to the city. They were once a short time used to transport decorations to the stage and to cool the theater with air from the river level.
And that horse? It`s not a Trojan Horse or any other “Greek Gift”. We’re underground of the theater – so it’s “just” part of the scenography.