Naturally occurring wormholes connect distant black holes.

Physicists know how a wormhole works but have never actually used one. Do you remember how a frictionless roller coaster works? That is, you start high and slow then you go down and fast but when you come back up again to the previous height you return to your previous slow speed? Well, the wormhole is the most efficient roller coaster you can imagine. Energy is not required to move through the wormhole, that is, you can simply switch off your rockets. Gravity accelerates you and pulls you through and then expels you the other side. You will feel like being swept by waves at the beach; nothing more. On your way through, gravity causes your clock to run slower and your ruler to shrink. Distant observers see you accelerating to relativistic speeds; as a result, your energy (and mass) increase dramatically in their view; but when you get out the other side everything returns back to normal (your clock, ruler...).