The best replacement for caves is caverns.

The best alternatives to caves are rated as following:
★★★★★  caverns
  lost river
  lost river gorge

Your alternative for caves is not listed? Suggest!
Speaking about caves, one probably shouldn't forget about waterfalls.

Other search terms related to caves and their ratings are:
★★★★★ waterfalls
You have in mind a companion for caves and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of caves in all years. Click on graph for more.
caves
caverns
lost river