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Caldeira Velha
Natural Monument
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With 13 hectares, the Caldeira Velha Natural Monument is located on the north side of Serra de Água de Pau, near the Lagoa do Fogo.
Caldeira Velha occupies an uneven relief area, surrounded by imposing scarps and has a deeply embedded fault valley, where a zone of secondary volcanism takes place, comprising a fumarolic field and an iron-rich thermal spring. This spring pours from a fracture at the base of a thick trachytic lava flow, currently with temperatures around 32 °C, while at the small dam near the waterfall, the water temperature is between 25 °C and 28 °C. Downstream are thermal pools where the water is heated by the fumarole (reaching temperatures of about 38 °C), making it possible to enjoy a relaxing thermal bath.
Although intensely changed regarding to vegetation, where prevail invasive alien species like Australian Tree Fern (Cyathea cooperi), Australian Blackwood (Acacia melanoxylon), Sweet Pittosporum (Pittosporum undulatum) and Kahili Ginger (Hedychium gardnerianum), in this area is possible to observe some flora specimens endemic of the Azores, such as Azorean Heather (Erica azorica), Azorean St. John's Wort (Hypericum foliosum), Azorean Holly (Ilex Azorica) and Azorean Ivy (Hedera azorica), and native species, such as Scotch Heather (Calluna vulgaris) and Royal Fern (Osmunda regalis).
Regarding the birds you can see the Common Buzzard (Buteo buteo rothschildi), the Common Chaffinch (Fringilla coelebs moreletti), the Azores Grey Wagtail (Motacilla cinerea patriciae), and the Azores Blackbird (Turdus merula azorensis). In the stream waters it is possible to see the Perez's Frog (Pelophylax perezi).
The Caldeira Velha Environmental Interpretation Centre displays information about the Azorean natural heritage and the thermal and geothermal phenomena of São Miguel island.
This area abuts part of the outside boundary of the Serra de Água de Pau Protected Area for the Management of Habitats or Species and is classified as a Ramsar Site under the Ramsar Convention.