Nimara Cave lies on Yıldız, often called Heaven Island, across the water from Marmaris. The cave has archaeological significance, with finds suggesting ancient ritual use, but the trip is also about the setting: pine forest, rocky slopes, and a break from the resort strip.
The cave itself is modest, so go for the combination of walk, views, and prehistory rather than expecting a vast cavern. Good shoes help, and the visit makes most sense as part of a half-day exploring the peninsula.