(The Doctor sees the large sphere)
The Doctor: This is a void ship.
Yvonne Hartman: And what is that?
The Doctor: Well, it's impossible for starters. I always thought it was just a theory. It's a vessel designed to exist outside time and space. Travelling through the void.
Dr Singh: What's the void?
The Doctor: The space between dimensions. There's all sorts of realities around us, different dimensions, billions of parallel universes all stacked up against each other. The void is the space in between. Containing absolutely nothing. Imagine that. Nothing. No light, no dark, no up, no down. No life. No time. Without end. My people call it the void, the Eternals call it the Howling. But some people call it hell.
Dr Singh: But someone built the sphere. What for? Why go there?
The Doctor: To explore. To escape. You could sit inside that thing, an eternity could pass you by. The Big Bang, end of the universe, start of the next… wouldn't even touch the sides. You'd exist outside the whole of creation.
Yvonne Hartman: You see? We were right. There is something inside it.
The Doctor: Oh yes.
Dr Singh: So how do we get in there?
The Doctor: We don't. We send that thing back into hell.