The Marshalls are brought to the mysterious world by means of a dimensional portal, a device used frequently throughout the series and a major part of its internal mythology. This portal opens when they are swept down a gigantic 1,000-foot waterfall. In "Circle", which explains the time paradox, this portal is actually opened by Rick Marshall himself, while in Enik's cave, as a way for the current Marshalls to return to Earth, resolving the paradox and allowing Enik to also return to his time.