Location from EarthBound by Shigesato Itoi / Nintendo
A treacherous swamp in southern Scaraba where Master Barf lurks, the Tenda hide, and wading into the deeper water slowly kills you.
The Deep Darkness is reached using the Yellow Submarine from inside Dungeon Man, and it's one of the most dangerous areas in the game. The swamp gets progressively deeper, and wading into the dark water drains the party's HP steadily. The Hawk Eye item is needed to see in the darkest sections. Enemies include Hard Crocodiles, Demonic Petunias, and Manly Fish. Master Belch, now Master Barf, ambushes the party here but is destroyed when Poo returns with PSI Starstorm. The Tenda Village is hidden in a cave here — a community of creatures so shy they can barely speak. Monotoli's helicopter (stolen by Pokey) is found abandoned with its engine missing. The Deep Darkness is where EarthBound stops being charming and starts being genuinely oppressive.
Dense, dark marshland. Murky water that deepens dangerously. Twisted trees and thick vegetation block the sky. The deeper sections drain HP with every step. Monkey villages and a hidden cave with the shy Tenda tribe.