Comet Cave connects Route 4 and Route 5. Travelers going from Burole Town and Amatree Town to Rochfale Town must pass through the cave. Like previous caves encounters, Comet Cave is dark and winding, and you can only see a few steps ahead of you.
The first time you visit Comet Cave, wild Pokémon will attack you two at a time, and you will fight alongside Theo, who will heal and revive all of your Pokémon at the end of every battle. This is a perfect time to level up your party. Beware, at the end of the cave you will encounter a dangerous Terlard. But keep in mind that it is possible to catch the Terlard. It will temporarily block the path behind you with Strength boulders, but after leaving and returning, the boulders will have vanished.
Immediately to your right when entering from Route 4, there is an underground river. While it cannot be crossed on your first visit, returning later with Surf, it can be crossed, leading to a secluded room with two Fossils.
This Fossil Room is also accessible from Route 5; coming from this direction requires Strength.
After defeating a trainer, you can choose to take either the Hair Fossil or the Tusk Fossil.
Take the fossil to Professor Cypress' Laboratory in Rochfale Town to revive either Slothohm or Snopach, respectively.
The following are the Pokemon that Theo brings that fight alongside the player when first entering Comet Cave. You are not penalized for attacking them, so you can KO them to ensure a shiny Pokemon's safety should you encounter one.