Wormholes are theoretical ideas dreamed up by physicists. They do not actually exist as far as we know, so we really do not know much about them. The only information we have are mathematical theories thought up by physicists. Nobody has ever seen a wormhole. We have no actual evidence of wormholes of any kind. Nobody knows how to create a wormhole. It's unlikely anyone could survive passage through most imagined wormholes since, in theory, gravity becomes infinite and you'd be crushed.
If wormholes exist, they might be found inside black holes. Yes they would have extremely large gravities, almost infinite. The idea for wormholes first came in 1935 when Albert Einstein and Nathan Rosen realized that the theory of general relativity allows the existence of “bridges” through the fabric of "space-time" connecting distant regions of time or space. In theory, by journeying through a wormhole, you could travel faster than a beam of light, or you could move back and forth in time. However, nobody has ever done this, and all the theories describing mathematically how this could be done require non-existent concepts like "imaginary time" or "negative energy matter", things that have never been found or observed in nature, or anywhere in the known universe. Also, all the theories proposing the creation of wormholes require vast amounts of energy, much more than is available to human beings. For further reading on wormholes try ThinkQuest.