The Sunken Island

This giant circular coral reef has a number of sandbars that are exposed at low-tide. At one point in the distant past this area was another island in the Alphatian archipelago but some calamity caused it to disappear into the sea.
  At the centre of the Sunken Island a deep pit (an ancient magma tube) draws water from the depths creating a swirling maelstrom at the centre of this part of the sea. A couple of trenches that cut through the coral circle expel water out into the seas. The trench expelling water into the Deep of Monktane is icy cold, so cold it contains shards of ice. The trench into the Waters of Cardtos is pure containing no salt, this expulsion creates a strange river of pure water that travels through the Waters of Cardtos till it dissipates near Sardinia. The pair of trenches that expel water into the Seas of Hepstar bring with them many aquatic creatures from distant parts of the world.