From 1865 to 1875 Vilmos Zsigmondi drilled a single hole under the City Park in Budapest. It went down 975 metres, almost a kilometre, and the hot water at the bottom of it is the reason the park has a bath. Széchenyi Gyógyfürdő, a neo-Baroque palace designed by Győző Czigler, opened on 16 June 1913. In 1938 a second well found water at 1,256 metres, coming up at 77 °C.
Look at the name over the door. Gyógyfürdő means medicinal bath. The water is thermal, obviously, since it arrives hot enough to scald. And according to Wikipedia it carries calcium, magnesium, bicarbonate, sulphate and fluoride, which makes it mineral too. Széchenyi happens to be all three at once. Most places are not, and the three words, used on spa signs as if they were synonyms, answer three different questions.
What makes water thermal?
Heat, and only heat, measured where the water leaves the ground. Not in the pool: the pool may have been cooled, or warmed with a boiler, and tells you nothing about the spring.
Where the line sits depends on the country. In Germany and Austria groundwater counts as thermal water when it reaches the surface at more than 20 °C; the German definition is set by the Deutscher Heilbäderverband, the spa association. Hungary sets the bar higher, at more than 30–31 °C, and has an old word of its own for such water, hévíz. It is also the name of a small town about eight kilometres from Keszthely, built around a warm lake.
Twenty degrees sounds cold for a bath, and it is. The threshold says where the heat comes from, the ground rather than a gas burner. It promises nothing about comfort. 249 baths publish a spring temperature of 20 °C or more, and 232 of them come up warmer than 30 °C, so they would pass the Hungarian line as well. Most operators publish only the temperature of their pools, so that count is a floor rather than a census.
A lake that is a spring
That lake is the word in a form you can swim in. Hévízi Tófürdő is a lake of about 47,500 square metres fed by two springs in a cave 38 metres down, and, according to Wikipedia, its whole volume is replaced every three and a half days. The bottom is a bed of peat up to eight metres thick. The red water lilies on the surface are not native: Nymphaea rubra was introduced by Sándor Lovassy in 1898. The operator gives the lake as 35–38 °C in summer. It is warm because the ground under it is, and nothing else.
Hot enough to need cooling
97 baths publish spring water at 50 °C or more, which no one could bathe in as it comes. At Parktherme Bad Radkersburg, in Styria near the Slovenian border, the operator describes water rising from a depth of two kilometres at 80 °C, under a pressure of 17 bar, and it has done so since 23 January 1978. Before anyone gets in, it is cooled to 34–36 °C.
A brochure that says 80 °C and a pool that feels like a warm bath are both telling the truth, about different water. If you want to know whether you can sit still in the basin for half an hour, the number you need is the pool temperature, a separate line on the operator's page.
Bath, in England, shows the same thing at a smaller scale. The operator of Thermae Bath Spa says the water fell as rain about ten thousand years ago, sank to around two kilometres below the surface, and comes out of the springs at 45 °C on average. Wikipedia describes the springs as the only naturally hot, mineral-rich waters in the UK. Bathing in them stopped in 1978 and resumed only on 7 August 2006, when the spa by Grimshaw Architects opened. Its open-air rooftop pool holds the water the Romans used, cooled to something you can stay in.
When is water mineral?
When something is dissolved in it, and the definitions disagree as soon as you ask how much. The figure people half-remember, a gram per litre, appears in the German description of a medicinal water: at least one gram of dissolved minerals or trace elements in every litre, or particular ions above guideline values. In the United States, the Food and Drug Administration calls water mineral from 250 parts per million of total dissolved solids. In the European Union, bottled water may be called mineral water when it is bottled at the source and has undergone minimal treatment: as Wikipedia summarises it, a rule about how the water is bottled, not how much is in it. Hungarian usage simply classes anything under a gram per litre as low.
So "mineral" on a sign is a claim about content, usually without a number. When there is one, read it. The operator of Terme di Saturnia, in southern Tuscany, traces its water back to the rain: it falls on the slopes of Monte Amiata, travels underground for about forty years, and comes out sulphurous, at 37.5 °C, close to body temperature. It gives the total dissolved minerals as 2,790 mg/l and the hydrogen sulphide as 14.5 mg/l, which is why the place smells the way it does. In the local legend, the god Saturn's lightning opened a crater and hot water poured out of it.
At Bad Radkersburg the operator gives about 8,000 mg per litre, eight times the German gram. Neither number says anything about temperature, and a water can be heavily mineral and cold. The springs of Luhačovice, in Moravia, show it. The first written report on the chemistry of the water there was published in 1669 by a physician, Jan Ferdinand Hertod z Todtenfeldu. The Serényi family were the first to make use of the springs and built the spa; one of them, adjusted around 1760, took the name Vincentka in 1792 and has been sold in bottles since 1820. Wikipedia describes it as heavily mineralised, naturally effervescent residual seawater, and lists diseases of the vocal cords and airways among its indications. The spa there today is Lázně Luhačovice.
Who decides that water is medicinal?
A state does, about one source at a time. Of the three words, it is the only one that is not a measurement.
In Hungary, a decree of 1999 defines gyógyvíz as a natural mineral water with a proven healing effect, and the national spa authority grants the name. Bathing water has to meet the requirements for external use and keep a constant chemical character. In Germany a Heilwasser is, in law, a medicinal product: bottled, it needs an official approval like a medicine, although water used on the spot for bathing or inhalation is exempt from that approval. Neither definition says anything about heat. A medicinal source can be cold, and some of the most famous are.
Mariánské Lázně, in western Bohemia, has around a hundred mineral springs, and Wikipedia gives them as cold, carbonated and often rich in iron, at 7–10 °C. People came anyway. In Nové lázně, rebuilt in 1893–1896, the operator keeps the Roman Baths, two original mineral-water pools among marble columns, and the Royal Cabin built in 1896 for Edward VII, with a scale hidden in a chair. Not far away in Františkovy Lázně the springs run at 9–11 °C all year and the town heats the water for its baths. The springs there mattered enough to start a fight: in 1791, according to Wikipedia, the women of Eger, today's Cheb, who made their living scooping, carrying and selling the water, tore down the pavilion and basin that the doctor Bernhard Adler had built over the Franzensquelle. They felt their water-bearing rights were threatened.
Back on the warm side, the words separate clearly. Of the 249 baths whose spring comes up at 20 °C or more, 92 also use water recognised as medicinal, which is 37%. For the rest no such recognition is stated: they are warm, thermal by the German definition, and that is all the word promises. Our piece on certified medicinal water goes through how the designation works country by country.
What the recognition certifies is the water: its composition, its origin and that an authority has examined it. It is not a finding that a bath in it will treat any particular condition. When an operator lists what its water is good for, that is the operator's statement. If you have a heart or circulation problem, high blood pressure, or are pregnant, ask your doctor before booking a course of medicinal baths.
What is a first visit to a medicinal bath like?
More regulated than a leisure pool, and the rules are worth reading at home.
Friedrichsbad in Baden-Baden is the ceremonial version. Karl Dernfeld designed it in the Renaissance style and it was built between 1869 and 1877; the excavation turned up Roman baths, still on show in the building. Its water comes from twelve sodium chloride springs at 56 to 68.8 °C. The bath combines Roman bathing with the Irish hot-air bath, so you move through dry warm and hot air, mild steam and then thermal pools at different temperatures. The operator suggests about three hours and sets no time limit. Two things surprise first-time visitors. On five days a week, Monday, Tuesday, Thursday, Friday and Sunday, bathing is without swimwear, and men and women bathe together; Wednesday and Saturday are swimwear days. And children under fourteen are not admitted. Wikipedia records a line attributed to Mark Twain: "After 10 minutes you forget time, after 20 minutes you forget the world." Our guide to textile-free bathing explains the custom.
At Hévíz the operator is specific about time. It recommends 20–30 minutes in the lake at a time, breaks of at least half an hour, and no more than an hour and a half a day; without a doctor's advice, thirty minutes. The lake is not recommended for children under twelve, and the house rules ask people with cardiovascular disease or high blood pressure, and pregnant women, to consult a doctor first.
Budapest has its own rule. Under a 2023 government decree, the city's bath company says, children under fourteen may not use the medicinal pools without a medical prescription, and Széchenyi's own visitor FAQ points families towards the city's lidos, Palatinus and Csillaghegy among them. Our Budapest guide covers both kinds of bath.
Which word should you look for when choosing a bath?
If you want an afternoon of sitting still in warm water, thermal is the word, but the number to check is the pool temperature. A spring at 80 °C and a basin at 36 °C are both normal; a spring at 22 °C feeding an unheated pool means you will swim, not sit.
If you are curious about the water itself, mineral is the word, and the useful question is what. A sulphur water smells, an iron water stains the stone orange, a radon water is a matter for a doctor's advice. Our pieces on sulphur water, iron water and radon water go into each.
If you are thinking about a course of treatment, medicinal is the word that has a legal meaning, and the conversation to have is with a spa doctor and your own doctor rather than with the ticket desk. And if you have been wondering whether a "Therme" has to have spring water at all, what Therme means answers that.
The list under this article holds the 249 baths whose water reaches the surface at 20 °C or more: the first of the three words, the one that measures heat. It takes in springs as hot as Széchenyi's 77 °C and Radkersburg's 80 °C, cooled before anyone gets in, and Saturnia's water at about body temperature. The cold medicinal springs of Bohemia are not on it, which is the whole point.