The Burren, Ireland
I first encountered word karst in Ireland, where the bare rocky plateau in County Clare known as the Burren was said to be an example of karst terrain. On our visit to Thailand in 2005, we saw high limestone pillars rising from the water of Phang Nga Bay and from the land on the nearby coast; we had also seen some sticking up among the country’s northern mountains. Anyone who has looked at old Chinese paintings (or seen Crouching Tiger, Hidden Dragon) has seen them there, too. The geological term we learned to apply to these lofty structures was also karst.

Phang Nga Bay, Thailand
The Burren plateau was smoothly rocky, nothing like the needle-pointed heights in Thailand. But what both had in common was that they were made almost entirely of limestone, among the softest and most water-soluble members of the mineral kingdom. Karst terrain is typically full of caves made by rivers flowing beneath the surface. The pillars we saw in Asia were what remained of some prehistoric karstic plateaus that had at some time in its history been surrounded or covered by water. Later the water sank or the ground rose, and the parts that, for one reason or another, hadn’t been dissolved were left standing. Fresh or salt water kept working at these survivors; many of the pillars in Phang Nga Bay had been hollowed to the point where the top caved in, leaving an open lagoon in the middle.

Slovenia’s Kras is flatter, like the Burren, except that much of its limestone has arable land on top. (If you go to the northern part, however, where Italians and Austrians fought bloody battles in World War I, or head a few miles farther south into western Croatia, you can find enough bare limestone to satisfy the most irrational enthusiasm for such terrain.) Below the surface, rivers have been carving their paths for eons, creating all those caves.

Underground Rivers

Among the other geophysical phenomena of Kras are rivers that burst from underground and, after flowing for a few miles beneath the sky, plunge back in. A notable example of this type of river is Ljubljana’s own, the Ljubljanica. It’s said to have been given seven different names, each belonging to a segment of the river that disappears below ground — except for the last, which runs through the city and flows into the Sava just beyond it. Because the underground connections were out of sight, no one knew until relatively recently that the parts above ground belonged to one river.

In my Internet researches, I found many iterations of this description of the Ljubljanica, but it was very hard to find further information, such as what its other six names might be. Eventually, however, I found a site that said, “At its first source under Mt Snežnik it is called Trbuhovica, then Obrh, Stržen, Rak, Pivka and Unica, until [it comes to] the surface as the Ljubljanica river.” Count ‘em: seven names. I actually managed to locate all but the first (which is probably too small to be labeled) using Google Maps, and the last and largest three (starting with the Pivka) are also visible on my largest-scale (2km/cm) paper map.

But another website made me wonder if I should trust this list: “[The Ljubljanica] has two karstic sources — the Cerknica and Pivka rivers, which converge in a subterranean confluence deep within the Planina Cave (Planinska jama), a few kilometres to the northeast of Postojna.” Problem: the Pivka is on the list, but the Cerknica (which is close to, and has the same name as, Slovenia’s biggest disappearing lake) isn’t.

The Google map wasn’t much help; it showed the Stržen, which is on the list, merging with the Cerknica, but both streams seemed to end by going underground (at what point I couldn’t determine, since the map didn’t indicate what direction they were flowing in). Later, though, I found an official site describing the Planina Cave, whose main feature is the underground confluence of two rivers (the largest underground confluence in Europe, by golly), and it says clearly that the rivers are the Pivka and the Rak, not the Pivka and the Cerknica. So it would seem that the merged Stržen and Cerknica find their subterranean way into the Rak, and since the Cerknica contributes a lot of water from sources other than the streams in the list, the second statement is as true as the first.

Intermittent Lakes

The porosity of karstic terrain and the presence of so many underground rivers account for another phenomenon of the Kras district: lakes that fill up in the spring and disappear almost completely in the summer. The largest and most famous in Slovenia (and certainly in Europe — perhaps in the world) is Cerknica Lake (Cerkniško jezero), which is a lake from October to June and a plain the rest of the time. The ancient Greek geographer Strabo knew about this lake, but was at a loss to explain it.

The mechanism is another typical karstic feature: sinkholes that lead down from the surface to underground waterways. During the cold seasons, rain and snow pours such quantities of water into these channels that it backs up through the sinkholes, like dishwater from a sluggish drain, and spreads out over the land. In warm weather the increase in sunshine and decrease in precipitation reduce the subterranean pressure, and the water runs back down the sinkholes, turning the lake bottom into dry (or comparatively dry) land again.