El Hierro's famed Pozo de la Salud (Well of Health) was long known for miraculously healing local cattle, but has been recognised for its mineral-rich healing properties since the 18th century (in the mid 19th-century its water was even exported to New York and Cuba). You can walk down to the small seaside well, whose waters are said to cure a variety of ills, though it’s generally closed up. It's 3.5km northwest of Sabinosa off the HI500.
If you fancy taking the waters, book in for a stay or treatment at the adjacent Hotel Balneario Pozo de la Salud.