The smallest and most remote of the inhabited Canary Islands, and the DO most likely to produce a wine you have never tasted and cannot find anywhere. El Hierro was designated a UNESCO Biosphere Reserve in 2000. The wines are tiny production, rarely exported, and made from varieties that exist almost nowhere else.
The island is essentially a massive volcanic shield, with dramatic cliffs and microclimates ranging from arid south to humid north. The wines reflect this extremity. Whites can be strikingly mineral and saline. Reds from Baboso Negro are deeply coloured and intensely flavoured -- concentrated in a way that reflects the struggle of the vine.
Finding El Hierro wine outside the island is genuinely difficult. The production is tiny and most is consumed locally. If you visit El Hierro (and it is worth visiting), drink as much as you can. Otherwise look in specialist Canarian wine shops on Tenerife.