The Melmond area is covered in swamps because the earth is rotting. (This is an example of Mr. Doty's other favorite word, irony. Swamps, or wetlands, are a very healthy and important aspect of nature, at least according to the Magic School Bus. A more appropriate situation would be if the land was all becoming desert.)
Melmond is the first place you should be going. Learn of their troubles and pick up some new armor and spells.
A titan lives in the appropriately named Titan's Tunnel. He blocks your passage, but he has a sweet tooth.
The Earth Cave seems to be the source of the earth's rot.
After the Titan graciously lets you by you can reach Sarda's Cave.