Look for 0-SphereIICore/Config/blocks.xml:
<property name="CaveEnabled" value="false"/>
<property name="CaveType" value="Random" />
<!-- Mountains, Random, All, DeepMountains -->
Random will spread them outrandomly, some with cave openings. If you just want to play around, you can change that to All, and every chunk will have a cave. You'll have to dig through to it, as there's no opening yet; but it should only be a few blocks below terrain.