"O-okay mom, seems like a long swim..."
"We shall be fine, now take a breath."
After the exodus to the surface, the long forgotten underground was pretty much left abandoned except for a select few deciding to stay, but the ruins were pretty much forgotten about and after a few freak storms caused by some unknown magic, the ruins became flooded with water leaking from the mountains and eventually filled up.
Toriel and Asriel were exploring the underground to see if they could salvage whatever they had back when they lived underground, but when they found it to be almost completely flooded, they had to strip down and continue their search swimming deep underwater. Asriel was a good swimmer but certainly wouldn't last without his mom's help, so latched onto her back as she dove under the cold surface and began kicking into the ruins.
Everywhere they looked was strewn rocks and pillars, memories of a bygone era washed away, Toriel feeling the cold water hug her body tightly, her nipples standing on end as she felt her son's paws hold onto her shoulders.
What they accounted for was that they would be diving down into deep, cold and murky waters.
What they didn't account for was for how long.
The ruins themselves had been flooded so much that there wasn't a lick of air for ages on end, and as Toriel descended another flight of flooded stairs and with how much she and her son were struggling to hold what little air they had left, she realised that she might have misjudged the situation.
As she swam ahead, picking up the pace as her breasts bobbed back and forth, Asriel let out a strained gasp, letting go of his air in one big plume of bubbles, "Hrrrglblaaa!!"
He sank down on her mother's back as she looked at him with panic, her cheeks flushing a deep purple as she swam even quicker, straining further as she kept swimming in the vain hope that she would find a reprieve soon.
"Hold on, Asriel. Mommy's gonna get you out of here, I promise no matter what happens, we will be alright."
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