One interesting fact about El Hierro is that it was once known as the Meridian Island (Isla del Meridiano) because it was considered by some to be the westernmost part of the Old World and thus a logical place for the Prime Meridian (longitude 0°).
The British of course, disagreed placing the Prime Meridian at the Royal Observatory in Greenwich, the location we use today.
The westernmost end of El Hierro
The western edge of the world prior to the 1400s.
The Canary Islands (Islas Canarias), by the way, have nothing to do with canaries. The name derives from the Latin Insula Canaria meaning "Island of the Dogs" due to either to numerous dogs on the islands or, some believe, seals (canis marinus or "sea dog" in Latin).
The islands had an indigenous population, since at least neolithic times, apparently related to the Berbers of North Africa. Greeks, Phoenicians, Carthaganians, and Arabs all visited at various times. Starting around 1400, the islands came under the influence of Spain and ships later bound for the New World found it a convenient place to stop before catching the northwest tradewinds for the Caribbean.
The Canaries are volcanic islands which formed, like the Hawaiian Islands of the Pacific, by the passage of the African Plate over the Canary Hotspot - a deep plume of hot material in the mantle which apparently formed around 60 million years ago.
One of the volcanic islands, El Hierro, is currently active. With a triangular-shape, and rising 1,500 meters from the ocean surface, the island is pockmarked with hundreds of craters from volcanic activity. The island has also been affected by at least 3 massive landslides. The largest of these was the El Golfo landslide which shaped the nothern shore of the island around 15,000 years ago and dumped hundreds of cubic kilometers of debris into the ocean (almost certainly causing a tsunami). You can clearly see the scar of this event in the image of the island below.
Radiocarbon dating places the last large eruption on El Hierro at around 550 BCE. Last summer, however, thousands of tremors signalled the movement of magma in the subsurface. This led to the formation of a fissure on the seafloor 1 km south of the island with continuing volcanic activity.
Pretty cool. What we're basically seeing is the slow formation of a new island, or the growth of El Hierro into a larger island, in the Canary chain. The gases and lava erupted on the seafloor are killing fish ad causing some disruption in the lives of the islanders, but that's the price you pay for living around volcanoes.
Will it get worse? There's always the possibility things will ratchet up a bit there (the islanders are apparently packed and ready to evacuate if need arises). Earthquake swarms are still occurring indicating the movement of magma but relatively small amounts and deep (about 12 km).
It's certainly a place to watch and will likely remain active for years to come.