"The first wonder is the lake Lumonoy. In there are sixty islands, and men dwell there, and sixty rocks encircle it, with an eagle's nest on each rock. There are also sixty rivers flowing into that place, and nothing goes out of there to the sea except one river, which is called Lenm/Lenin."1,2
This wonder is a geographical one. Britain was once a land of lakes and isles. The draining of the marshes of the Vale of York, the Fens, and southern England, and the filling of many of our lakes with sediment since the end of the last ice age ~18-10 thousand years ago has left only Scotland, Cumbria and the Broads as lakeland areas. But these still inspire thousands to come and marvel at their lakes and islands, so it shouldn't be surprising that a lake would also be in a dark age list of wonders. Loch Lumonoy is likely to be Loch Lomond3,4, which drains south via the River Leven into the mouth of the River Clyde, and which is famous for the large number of its islands.
You may like to read about the the Myth and Science of this Wonder.