Those original NES square waves and noise channels, cleaned up but still alien and cold. The 1987 Metroid soundtrack basically invented what isolation sounds like in a video game. Hirokazu Tanaka built an atmosphere out of hardware limitations.
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"Brinstar" is the one everybody knows, that bouncy bassline that somehow makes an alien planet feel almost welcoming before the dread sets in. "Kraid's Lair" is the opposite, slow and suffocating, just droning tones and silence where melody should be.
"Tourian" strips everything down to pure unease. The Famicom Disk System versions of the title and ending themes add this slightly different tonal quality. Warmer, somehow. If you've never actually sat and listened to this outside the game, the tension hits completely different through headphones.