We sink into Stargate, sweeping the void with our dive lights. Fifty feet from the surface looms a pale haze, less smoky than fibrous, like a silvery net of faint, swirling cobwebs hovering motionless in the darkness. It's a layer of hydrogen sulfide, a toxic gas created by bacterial colonies and decaying organic matter. Divers entering the gas may experience itching skin, tingling, or dizziness; some smell rotten eggs as it penetrates their skin and metabolizes through their lungs. The gas density in Stargate is relatively low, but I'm struck by a wave of nausea as we descend. I glance at my guide, Brian Kakuk—one of the world's foremost cave divers. He appears unfazed. My head begins to throb; clearly, I'm unusually sensitive to the toxin. In the epic poem Beowulf, "dim serpent shapes" in the depths guard the lake of Grendel and his mother, shielding their lair. The otherworldly mist in Stargate appears to serve a similar role—a poisonous curtain that protects the deeper reaches of the cave.