Red Mountain is a location on Angel Island.
Knuckles the Echidna first ventured through the place to hunt for the Master Emerald shards after Dr. Eggman released Chaos. E-102 Gamma also ventured through to rescue his fourth E-100 Series brother Epsilon. (SA/SADX)
Eclipse the Darkling and his Dark Arms crash-landed in the Red Mountain Zone following the destruction of the New Black Comet. It briefly served as their dwelling, until Team Dark and Knuckles the Echidna began looking for them. After Eclipse departed the island, G.U.N. forces arrived to remove his ship's wreckage under the supervision of Dr. Julian Snively. (SU: #67, #68, #69, #70