this is just a theory but what if the horneater oceans are portal to shadesmar on a reread i noticed that when rock described the god of mischief, who he sees in the oceans, sigzil recognizes the description, pointing toward him being hoid and he asks the year so it seems that he just came to roshar
The top of each mountain forms a kind of crater," Sigzil explained, "which is filled with a large lake of warm water. The heat is enough to create a pocket of livable land, despite the altitude. Walk too far from one of the Horneater towns, though, and you'll end up in freezing temperatures and ice fields left by the highstorms."
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"Is not just water," Rock said. "Is water of life. It is connection to gods. If Unkalaki swim in it, sometimes they see place of gods."
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"Place of the gods?"
"Is where they live. The waters of life, they let you see place. In it, you commune with gods, if you are lucky."
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"You can see the spren," Kaladin pressed, "because of what happened to you in these waters."
"Is not part of the story," Rock said, eyeing him. "It is... involved. I will say no more of this thing."
"I'd like to visit," Lopen said. "Go for a dip myself."
"Ha! Is death to one not of our people," Rock said. "I could not let you swim."
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"Swimming in the emerald pools is death to outsiders," Sigzil said, "because you execute outsiders who touch them." "No, this is not true. Listen to story. Stop being boring."
"They're just hot springs," Sigzil grumbled, but returned to his drink.
Rock rolled his eyes. "On top, is water. Beneath, is not. Is something else. Water of life. The place of the gods. This thing is true. I have met a god myself."
"A god like Syl?" Kaladin asked. "Or maybe a riverspren?" Those were somewhat rare, but supposedly able to speak at times in simple ways, like windspren.
"No," Rock said. He leaned in, as if saying something conspiratorial. "I saw Lunu'anaki."
"Uh great," Moash said. "Wonderful."
"Lunu'anaki," Rock said, "is god of travel and mischief. Very powerful god. He came from depths of peak ocean, from realm of gods."
"What did he look like?" Lopen asked, eyes wide.
"Like person," Rock said. "Maybe Alethi, though skin was lighter. Very angular face. Handsome, perhaps. With white hair."
Sigzil looked up sharply. "White hair?"
"Yes," Rock said. "Not grey, like old man, but white- yes he is young man. He spoke with me on shore. Ha! Made mockery of my beard. Asked what year it was, by Horneater calendar. Thought my name was funny. Very powerful god."