The Canary Islands have almost no rivers, very little rain on the side where most people live, and a permanent population of over two million plus millions of visitors a year — so nearly every drop that comes out of a tap here has either been tunnelled out of a mountain or taken from the sea and stripped of its salt. It is one of the most impressive pieces of engineering in the archipelago, and it is completely invisible to the average holidaymaker.

There are two basic systems, and which one supplies your hotel depends on which island you are standing on. The high, western islands — Tenerife, La Palma, La Gomera, El Hierro — catch enough cloud and rain on their northern flanks to hold groundwater inside the rock, and they get at it by drilling horizontally into the mountainside. The low, dry eastern islands — Lanzarote, Fuerteventura, much of Gran Canaria — have almost no usable groundwater left and run instead on desalination plants that push seawater through membranes at high pressure.

Neither is cheap, and both explain things you will notice on holiday: why the water tastes the way it does, why a hotel asks you to reuse towels, why an apartment complex has a sign about not hosing the terrace, and why the supermarket aisle of five-litre bottles is the length of a bus.

Why there are no rivers

Volcanic islands are made of porous rock. Rain that falls on young basalt and ash does not run off in streams so much as sink straight through it, travelling down and sideways inside the island until it meets something impermeable. What look like rivers on the map — the barrancos that cut every island into wedges — are dry ravines that carry water for a few violent hours after a storm and nothing at all for the rest of the year. There are a handful of permanent trickles in the wettest corners of La Palma and Gran Canaria, and that is the sum of the archipelago's surface water.

What the islands do get, on their northern sides, is fog. The trade winds push a persistent cloud layer against the high ground between roughly 600 and 1,500 metres, and the laurel forests that grow in that band comb moisture out of it through their leaves — horizontal rain, farmers call it. Under the old heather and laurel canopies this delivered a real, measurable water supply, which is one of several good reasons the islands stopped cutting them down.

The galerías: tunnels inside a mountain

Tenerife's answer, from the late nineteenth century onwards, was to walk into the mountain and keep going. A galería is a horizontal tunnel, typically around two metres high and not much wider, bored into the flank of the island until it cuts the water table, at which point the rock begins to weep and the water runs out under gravity along a channel in the floor. No pumps, no electricity, just a very long hole in the right place.

The scale is genuinely difficult to believe. Tenerife has over a thousand of these tunnels, and their combined length is usually put in the region of one and a half thousand kilometres — a concealed network longer than the road system of the island it sits inside. They were dug by hand and by drill and explosive, by crews who worked in bad air for weeks at a time, and the work killed people. Many galerías are privately owned by water communities, shares in which have been traded between island families for generations.

This is why water rights, not land, were historically the real wealth on Tenerife and La Palma. A family could own a hillside and still be poor; a family with a share in a productive galería could sell water to everyone else. Some of those share certificates are still in circulation.

The problem with tunnels

Draining an aquifer works until it does not. Decades of enthusiastic galería digging lowered water tables across Tenerife and Gran Canaria, and many tunnels that once ran freely now produce a fraction of what they did, or nothing. Some have had to be extended deeper into the mountain to chase the falling water. On Gran Canaria the equivalent story involved thousands of wells and a network of reservoirs in the interior, and it ended the same way, with the island turning to the sea for the difference.

Lanzarote went first — in 1964

Here is the detail worth carrying away. Europe's first municipal desalination plant for public drinking supply did not open in some Mediterranean city; it opened in Arrecife on Lanzarote in 1964, a flash-distillation plant built because the island had genuinely run out of options. Before it, Lanzarote drank rainwater collected off painted catchment aprons into underground cisterns, and water was shipped in by tanker from other islands in bad years. Every traditional farmhouse on the island still has an aljibe under the yard.

Sixty years on, Lanzarote and Fuerteventura are essentially one hundred per cent desalinated. If you drink a glass of tap water in Playa Blanca or Corralejo, it was in the Atlantic last week.

How reverse osmosis actually works

Modern plants use reverse osmosis rather than boiling. Seawater is filtered, then forced at very high pressure — tens of atmospheres — against a membrane whose pores let water molecules through and hold back dissolved salts. Roughly four to five litres of seawater go in for every litre of fresh water that comes out; the remainder leaves as brine, roughly twice as salty as the sea, which is returned offshore through diffusers designed to mix it quickly.

The cost is energy. Desalination is one of the largest electricity consumers on the eastern islands, which ties the price of your shower to the price of fuel and makes the islands' push into wind and solar more than a green gesture. El Hierro is the poster child: its Gorona del Viento scheme pairs a wind farm with a pumped-hydro reservoir so that surplus wind can be stored as water uphill and released when the wind drops, and the island's desalination and pumping demand is part of what the system was sized around.

So why does it taste like that?

Reverse osmosis produces water that is chemically very pure and, to a British palate, oddly flat — almost sweet, with none of the chalky background of hard tap water at home. Plants add minerals back in and chlorinate the supply before it goes into the network, and the balance they strike varies by island and even by municipality. On the western islands, where supply is groundwater that has spent years in volcanic rock, you get the opposite: a distinct mineral character, sometimes faintly metallic, occasionally with a whiff of sulphur in the highest-mineral sources.

Neither is a health problem. Canarian tap water meets Spanish and EU drinking standards, and our fuller answer on whether you can drink the tap water goes through the exceptions — old building plumbing, rooftop tanks in apartment blocks, the occasional local notice — that are worth knowing about. Most residents drink bottled water at home not because the tap is unsafe but because they do not like the taste, which is a different argument entirely.

The islands' own bottled water

You will see the volcanic geology on the supermarket shelf. The village of Firgas in the green north of Gran Canaria has given its name to one of Spain's better-known sparkling mineral waters, drawn from springs in the hills above it, and the town has made a small tourist attraction of the water theme. Several other island brands bottle spring water from the high forest belt. It is worth trying at least one: it is the taste of rain that fell on a pine forest and then spent a long time inside a mountain.

Farming takes most of it

Households are not the biggest user. Agriculture takes the largest share of Canarian water, and bananas take the largest share of that — a plantation under plastic in the Aridane valley or the Orotava valley runs on a drip system fed from tanks that are themselves fed from galerías, wells or desalination. The network of channels, cisterns and hand-built terraces that shaped every farmed hillside was a water-distribution system as much as a growing one, and the economics of the island banana only work because that plumbing exists.

What a visitor should actually do

The bottom line

Every island you visit solved the same impossible problem in a different way, and the solutions are still visible if you know where to look: the tunnel mouths in the ravine walls above Tenerife's north coast, the whitewashed catchment slopes on Lanzarote, the squat blue-grey sheds near the coast that turn out to be desalination plants. On El Hierro they used to venerate a tree — the Garoé — that condensed enough fog to supply a settlement, and whether or not the legend is exact, it tells you how completely the question of water has shaped life here.

Understanding that changes how the islands read. A place that looks arid and looks prosperous at the same time is a place that has spent a century and a half being extremely clever about water, and the price of everything from a bottle of water to a hotel night carries a little of that engineering in it.