Long ago, a huge volcano erupted so powerfully that its top collapsed inward, leaving a giant bowl shape called a caldera. Over thousands of years, rainwater and groundwater filled the bowl to create Lake Coatepeque. Because the lake is so deep — up to 120 metres in places — sunlight scatters in the water and creates that famous brilliant blue colour.
The lake is surrounded by steep green hills, and small islands dot its surface. Local fishers paddle out in small boats to catch fish such as tilapia and black bass, which are an important source of food for families in the area. On weekends, many Salvadoran families come to picnic on the shore, swim in the warm water, and enjoy boat rides.
From the rim of the caldera — the high ground around the lake — you can look out across the whole shining circle of water with Santa Ana Volcano rising behind it. This view is one of the most photographed scenes in El Salvador. Some visitors say it feels like standing on the edge of a magical world hidden inside a mountain.
The lake is warm throughout the year because of geothermal heat from deep underground. Hot springs bubble up in a few spots along the shore, meaning the water there is naturally heated — like a giant outdoor bath. Scientists study these springs to understand the geology (the rocks and structures) beneath El Salvador.