The Cavern Pokemon evidence can be found at Ballimere Lake and Lakeside Cave. Once you collect 100% of the evidence, the rest of the footprints for that type of Pokemon will disappear. After you find 100% of the Cavern Pokemon evidence and report back to Sonia, she will tell you that the Cavern Pokemon can be found in Lakeside Cave! Travel to Lakeside Cave to find Terrakion (level 70) wandering around. After you catch Terrakion, return to Sonia to collect a reward.