Wormholes are holes in space and time that connect the Old Universe to the smaller Pocket Universe. Going through a wormhole from the Old Universe into the Pocket Universe is a one-way trip, since extreme gravity prohibits people in the Pocket Universe from going through a wormhole in order to access the Old Universe.
In some cases, spaceships and even entire planets and solar systems have passed through Wormholes into the Pocket Universe.