A mineral is not a city
There is real evidence of ancient continental material beneath Mauritius. In a 2017 Nature Communications paper, Lewis Ashwal and colleagues reported very old zircon crystals inside much younger volcanic rock. Their measurements support the presence of ancient continental crust and a reconstruction involving fragments associated with Madagascar and India. The paper also acknowledges uncertainty about the fragments' exact extent and geometry.
Those ages concern geological history on scales of millions and billions of years. They are not dates for human buildings, ships or settlements. The finding does not establish that people watched Mauritia sink. A story about an inhabited lost nation cannot use the word 'continent' to jump over that enormous difference in time. The Glasswater League is therefore an invented archipelago in a counterfactual Indian Ocean, not a human civilization supposedly proved by those zircons.
Sources: mauritia-zircons
A better source of human scale
Actual Indian Ocean ports offer a firmer comparison for economic life. UNESCO's account of Kilwa Kisiwani and Songo Mnara describes Swahili trading cities connected with Arabia, India and China, supported by surviving buildings and archaeological evidence. These real societies do not need a hidden predecessor to explain their achievements. Our invented league is not assigned credit for their architecture, navigation or commerce.
Seasonal winds provide another useful constraint. NOAA explains monsoons as seasonal shifts in wind associated with changes in heating over land and sea. For a fictional sailing port in a monsoon-affected part of the ocean, a missed departure can mean a costly wait. That is a stronger engine for a story than an unexplained fleet able to go anywhere whenever the plot needs it.
Sources: kilwa-unesco, noaa-monsoon
The invented league's balance sheet
Glasswater consists of three imagined islands with limited farmland and one dependable anchorage. Residents repair visiting vessels, supply water and trade preserved fish; imported grain covers a recurring shortfall. Rain-fed storage serves the crowded harbour quarter. Higher ground supports gardens and a less convenient second settlement. None of these locations is presented as a feature missing from a modern chart.
The league is an agreement about harbour fees, emergency food and maintenance, not a technologically superior civilization. Its weakness is concentration: the best landing, the market and the biggest storage houses occupy the same exposed shoreline. Damage to that shore becomes a political problem because moving trade uphill costs money before it earns any.
A lost place need not mean a lost people
In the story, a storm damages the harbour and a blocked channel keeps cargo offshore. The league debates whether to rebuild the old waterfront or pay for a smaller landing elsewhere. That local crisis is invented. It should not be confused with the separation of continental fragments in deep time.
A defensible historical investigation would need its own site evidence, dates and context. Here we offer a source-aware imaginative exercise: geology limits the claims, real ports supply comparisons, and the writer remains responsible for everything added between them.