It seems to be a location in which a certain river person would usually pick up someone who wanted to fair across the underground. A long time ago, when you were still nesting, the whole underground had a huge network of rivers which connected everything to each other, and the river person would always be there to pick you up.
It had 15 stops between lower snowy, and new home. And the lava wasn't a problem because the scientists at Royal sciences created the boat to be heat-resistant. But the service was slowly abandoned as more and more people wanted to just stay in their own place and avoid traveling across the underground. This led to the service only continuing between three major ports, Snowdin, Quiet Waters, and the science lab.
In your time as a Royal guard, you, on occasion, left your post open to talk to the river person, to learn about the history of monsterkind, and to get a friend. Your friendship was one of mutual distaste for your positions. You didn't like being a person who carries people over a small gap, and the river person didn't like that people refused to use the service anymore.
The two of you were really good friends, but when the barrier broke, they didn't want to leave their post, so you said your goodbyes, and you haven't forgotten about them. That is, until today, where you are about to reconvene with them. You ring the bell that usually signals a request of passage.