…not a word on your lips I bet. I had never heard the word before going to Vietnam. But we have a picture waiting for the word. The picture is of one of the magical elements in those extraordinary Chinese paintings: vertical hills loom up and outwards from mists, shacks tucked under their precipitous jutting brows, and the view completed by a hermit drinking or meditating under an ancient contorted pine seemingly writhing in distress at its own decrepitude.

A classic karst landscape near Cao Bầng in north-eastern Vietnam

These bizarre towers are karsts: tooth-like vertical hills, seemingly sketched from Chinese paintings, and which, although formed of soft limestone, rise upright for 500 feet and more. Some stand on flat plains either in convivial groups, or a little apart as more isolated spirits; others appear on high plateaux as voyagers venturing into new territories. Sometimes the groups crowd the landscape huddled in hundreds and seeming to advance like a school of outlandish giants lumbering slowly about their hidden business. In fact the word karst refers generally to any landscape that has been moulded by the effects of wind and (especially if there is a slight acidity in) the rain. They appear in many parts of the world and may show as simple decorated rock formations, larger basins, shafts, the disappearance of a stream, or the formation of limestone pavements - such as those in North Yorkshire. Here, in Vietnam and southern China, the world’s most spectacular examples are seen in the mature karst landscapes where most of the bedrock has been removed, leaving karst towers that can appear as ‘haystacks or egg-boxes’ and which may disguise huge underground drainage and cave systems.

A karst requiring the road to divert around it

These magnificent landscapes present us with hills typically festooned in rich cloaks of vegetation. This vegetation contrives to root to the inhospitable dry wind-swept monuments which are made from the remnants of corals and other deposits thrown upwards from the sea beds in one era, and then later weathered into the fantastic forms we now see. Although commonly 300 to 500 feet high, many reach twice that size, with razor-sharp crests atop sheer slopes, slopes which can average 90 degrees over their whole height. The pitted complex surface of the limestone, facing different directions and with different degrees of exposure, provides a range of habitats suiting many species in a very compact area. In some places water is trapped and acidic conditions develop, in other parts no water can be held at all, and the wind withers everything; and yet on the next face the setting is perfect for luxuriant growth. This diversity of environments allows more species, for any given area, than is found in the nitrogen rich, hot, wet soils of the surrounding countryside.

Karsts stand guard on the road to Pác Bó in Cao Bầng

While our image based on Chinese painting may be decorated with huts and hermits, they are not to be seen in Vietnam. Occasionally some human construction can be observed on a karst, but it is rare, and it is obvious why this is so on inspection, for the soft stone is continually crumbling and slipping, and of little use for supporting permanent structures. If there have been farming possibilities these are no longer in evidence, although certainly the resourceful Vietnamese will have gathered some crops from these precipices at times when the pressures of need were greater. The outside may offer humans rather few possibilities, but inside the soft weathered limestone creates endless spaces from crevices to underground caverns stretching for miles down into the ground. Some of these, in Vietnam, are so vast that we still have no idea of their extent, and they tantalizingly await exploration. Nearer the surface the caves formed are very familiar to humans and have been exploited for thousands of years. Excavations show that people first lived in them in the Red River area some 10,000 years ago, and the evidence gathered includes red ochre drawings and the tools left behind.

Karst formations at 2,000 metres near the Mã Pì Lèng pass at the northern tip of Vietnam

Jumping 10 millennia we hear from modern history about the use of such caves by the revolutionary forces under Hồ Chí Minh when he returned to Vietnam and lead the resistance to liberate the country from the occupying French. And more recently still, we can see examples from the American War where karst cave systems were of great use, such as those on Cát Bà island, where you may be shown a ‘hospital’ used in that war. This cave system runs through the hill with entrances on either side, and offers an even temperature, although possibly a little high for comfort, and extensive facilities including a very large cinema made from the central cavern. The quotes around hospital are necessary as this purpose does not seem quite credible given the distribution of rooms - allowing little space for patients considering the risk of infection, and appearing of much more value as a command centre. While such use of these great natural monuments may take us to the less attractive side of humanity, it is their aesthetics that catch our imaginations. Chinese art brought them to the attention of the West, but they are so ubiquitous here in North Vietnam, so outrageously exotic, and so astonishingly and unwontedly fertile, that they offer a fitting symbol of the Vietnamese countryside.