El Hierro has almost no sand, and the swimming here happens in volcanic rock pools instead — natural basins of clear Atlantic water, some of them tidied up with a ladder and a concrete rim, most of them not. If you come looking for a beach in the ordinary sense you will find perhaps one, and it is at the far end of a dirt road. If you come willing to swim in a lava pool with fish swimming past your knees, this is one of the best islands in the Atlantic.

The reason is geology. El Hierro is the youngest of the seven main islands and the least eroded, which means the sea has not yet had time to grind its lava down into sand. What it has instead is a coastline of raw basalt, much of it only a few thousand years old, full of hollows and channels and collapsed tubes that fill and flush with every tide. Islanders have used these for swimming for generations and the local authorities have gradually made a handful of them easier to get into.

The second reason is exposure. This is the westernmost and most south-westerly of the islands, it sits alone in deep water, and the swell arrives without anything to break it. A pool that is a millpond on a calm morning can be unswimmable by afternoon. That single fact shapes every piece of advice below.

La Maceta: the one everyone goes to

La Maceta, on the El Golfo coast below Frontera, is the island's best-known bathing spot and the fairest introduction to how this works. A set of natural pools has been linked and edged with walkways, steps and ladders, with a car park, showers and a small bar in season. On a flat day you swim in clear green water over black rock with the enormous wall of the El Golfo escarpment behind you, and it is genuinely lovely.

On a day with any northerly swell, the sea comes over the outer rocks and the pools become dangerous — and this is the point that visitors underestimate. People have been swept from these walkways. If the water is washing across the rock platform, do not go out on it to look. There is usually a notice; believe it. Our guide to reading the flags and the Atlantic applies with extra force on an island where help is a long way away.

The El Golfo pools face north-west, straight into the winter swell. Between roughly November and March, check the sea before you drive down — and check it again with your own eyes when you arrive. A morning forecast does not cover an afternoon set.

Tacorón: the sheltered alternative

Tacorón, on the south coast near La Restinga, is where you go when the north is rough — and it is arguably the more beautiful of the two. A series of small coves and inlets cut into a lava flow, with impossibly clear turquoise water, natural rock arches, a few picnic tables under thatched shelters and steps down into the sea.

The south coast faces the Mar de las Calmas, the "sea of calms", which is exactly what it sounds like: a stretch of water sheltered by the bulk of the island from the prevailing north-easterly, and one of the calmest pieces of open Atlantic in the archipelago. It is why the island's diving is concentrated here, and why snorkelling at Tacorón is a genuine pleasure rather than a fight. Bring a mask. There is no sand, there is no shade beyond the shelters, and there is no shop.

Playa del Verodal: the island's only real beach

Playa del Verodal is the exception that proves the rule — a wide open beach on the west coast below the Sabinosa road, reached by a steep track, with cliffs behind and nothing on it at all. What makes it remarkable is the colour: the sand here is reddish-orange rather than black, because the cliff feeding it is made of oxidised, iron-rich volcanic material rather than fresh basalt. Seen from the road above, against blue water, it is one of the strangest sights on the islands.

It is also, bluntly, not a swimming beach. The west coast is fully exposed, the shelving is steep, the currents are strong and there is no lifeguard, no phone signal in places and no quick way to get help. Go for the walk, the colour and the photographs. Get your swim somewhere else.

The pools of El Golfo, one by one

The El Golfo coast has a string of bathing spots beyond La Maceta, and locals have firm opinions about which is best on a given day. Charco Azul, a little further west, is a deep blue hole reached through a gap in the lava and entered by a flight of steps — the most photogenic of the lot. Charco Manso, over on the north-east coast near Echedo, is a rougher, wilder version. The Pozo de las Calcosas below Valverde has an old settlement of stone-and-thatch fishermen's huts above it and pools at the bottom of a long flight of steps.

What they share is that you must judge the sea yourself, that steps are steep and often wet, and that none of them is suitable for a toddler. The wider phenomenon — and it is one of the great pleasures of the western islands — is set out in our guide to the natural pools of the Canaries.

La Restinga and swimming from a harbour

In the far south, La Restinga is a small white fishing and diving village with a harbour you can swim in — an artificial beach and sheltered water inside the mole, which makes it the most reliably safe dip on the island and the one place families with small children can relax completely.

The village sits at the edge of a marine reserve, and the water is startling. Swim out a few metres from the harbour beach with a mask and you will see parrotfish, damselfish and often barracuda. In 2011 a submarine eruption opened on the seabed a couple of kilometres offshore, turning the sea green and forcing the village to be evacuated for a time; the vent is now a dive site and the marine life came back rapidly. It is a useful reminder that this island is still being built.

The meridian nobody remembers

Here is the detail worth taking home. For roughly three centuries, El Hierro was where longitude started. Ptolemy placed the edge of the known world at these islands, and in 1634 a French conference formally fixed the prime meridian on the westernmost point of El Hierro — the Punta de Orchilla — which put every longitude in Europe on a line drawn through this small, dry, almost empty island. It stayed in use in various forms until the international adoption of Greenwich in 1884.

You can drive to the spot. There is a lighthouse at Orchilla, a monument, and an enormous view of nothing but Atlantic — the next land west being the Caribbean. Combine it with the extraordinary wind-flattened junipers of El Sabinar and you have the island's best half-day that does not involve getting wet.

What to bring

Getting around

You will want a car, and you should expect the drive to take longer than the map suggests: the road from Valverde down into El Golfo loses a great deal of height in a short distance, and the western tracks are slow. Arrival is by ferry from Los Cristianos, a crossing of a couple of hours, or by the short hop on the small inter-island turboprops — see flying between the islands and the ferry network for how the connections work.

Where you sleep matters more here than on a bigger island, because crossing it takes real time. Our page on where to stay on El Hierro divides it the way locals do: south for divers, El Golfo for swimmers and walkers, Valverde for arrivals.

Before you go

El Hierro asks something of you that the resort islands do not. There is no lifeguard watching, no beach bar to retreat to, and no crowd to follow. In exchange you get water clearer than anywhere else in the Canaries, rock pools you may well have entirely to yourself on a Tuesday in October, and an island that has spent the last decade trying to run itself on wind and water power rather than diesel.

Go with respect for the sea and it is superb. If you are still weighing the island up, our honest verdict on whether El Hierro is worth visiting and the wider list of things to do on El Hierro will settle it — and if the rock-pool idea appeals but the isolation does not, the black-sand beaches of La Palma are the gentler introduction.