A stunning two-tone sea colour phenomenon appeared off the coast of Aberystwyth in west Wales in the UK.

The sea appears to be perfectly divided into two very different shades of blue at the harbour entrance where the River Ystwyth and Rheidol flow into Cardigan Bay.

The phenomena happens when colder fresh water with organic matter in it meets the warmer salty ocean in such a way that they do not immediately mix.

This only happens when the sea is calm and the rivers are high after heavy rain. You can see the clear blue sea divided at the exact point where the rivers flow in.

The sea appears to be perfectly divided at the harbour entrance where the River Ystwyth and Rheidol flow into Cardigan Bay at Aberystwyth in West Wales. The phenomena is caused when colder fresh water with organic matter meets the warmer salty ocean, causing them not to mix. This only happens when the sea is calm and the rivers are high after heavy rain. You can clearly see the clear blue sea divided at the exact point where the rivers flow in.