A crew of paleo-archeologists from Weaving Knowledge have been very, very carefully excavating a new site for the last several years. A large crew - this is exciting business - but often one with a rotating membership - still, new excited workers can be put to use pretty easily, and most of them aren't inexperienced with digs. It takes more than a year to get bored with an entire field, and jumping around between sites every season is common, eespecially for the adolescents doing most of the physical labor.
The site is located a twenty-one-fragment of a day's rowing off the coast where the Weaving River spills into the Moss Sea, before the Stone Islands that protect the relatively calm sea and the coastline beyond from the wider ocean. Underwater sites are hard, and very very highly seasonal - and very time-dependent, too, since it's best to wait for the low tide that comes at a schedule they need astronomers to time exactly - 'every fourteen hours, kinda' isn't a bad gloss if you're not doing long term delicate work in a field likely to be inudated when the tides come in, but they are, so they need to also take into account the drift over the course of the month as they spin around the Green Mother.
(They have sandbags and dikes and the like protecting the site, but those aren't absolutely reliable.)
It's ancient, is one thing that really quickly becomes apparent. Almost certainly precursors - which makes it well over a million years old, even before confirmation with more direct and accurate dating techniques. And it seems to be a rarely - and unusually intact - transport hub. Possibly part of how the precursors traveled to the stars...?
...And million year old technology from a lost civilization should not be active, which means they're very, very surprised when one of the enormous rings starts glowing after being fully excavated.
The inside stops showing the ground under it, and instead shows somewhere else...