Where Water Dissolves Stone: Ten Karst Landscapes

Begin with the simplest result of the process: more than 1,200 grass-covered conical hills on Bohol Island in the Philippines, each about 30 to 120 meters high. Rainwater and groundwater dissolved an uplifted marine limestone plateau along evenly spaced fissures and left the cones standing between them.

Near Guilin in southern China, hundreds of steep limestone peaks called fenglin, or peak forest, rise from flat river plains. The atlas records that caves first honeycombed the Devonian and Carboniferous limestone, and that their collapse left the isolated towers, which is why the Li River is often cited as the type example of this karst.

Ha Long Bay in Vietnam shows the same tower karst at a later stage, with roughly 1,600 limestone and dolomite islands and islets standing straight out of the sea. Dissolution has been so thorough that the terrain is reduced to isolated steep-sided towers, called fengcong or fenglin depending on how they cluster.

In Yunnan, rainwater dissolved Permian limestone preferentially along vertical joints, widening them into grooves and isolating tapering pillars, some more than 30 meters tall. This pinnacle or stone-forest karst is treated as one of the finest examples in the world and belongs to the South China Karst World Heritage property.

Madagascar's Tsingy de Bemaraha is a limestone plateau where needle-sharp pinnacles reach up to 30 meters. Water first carved caves and underground rivers along vertical joints, and as cave roofs collapsed the rock between the fissures was left as a dense forest of spikes, so the surface here records what happened underground.

Now go underground, to the plateau that gave the word karst its name. In southwestern Slovenia the Reka River sinks at Skocjan and has spent roughly two million years dissolving a passage through Cretaceous and Eocene limestone, making one of the largest known underground river canyons in the world.

On New Zealand's North Island, underground drainage has dissolved uplifted Oligocene-to-Miocene marine limestone over roughly the last two million years. The caves are best known for the Glowworm Grotto, where Arachnocampa luminosa larvae glow on the ceiling. The cave system was first explored and mapped in 1887 and has been open to visitors since 1889.

Carlsbad Caverns shows that the acid need not come from rain. Hydrogen sulfide rising from oil and gas deposits mixed with groundwater to make sulfuric acid, which dissolved the Permian Capitan reef limestone from below, produced unusually large chambers such as the Big Room, and left gypsum as a byproduct.

Off Belize, a sinkhole about 300 meters across and more than 120 meters deep began as a karst cave that formed on dry land when sea level was lower. It partly collapsed and flooded as the sea rose, and stalactites recovered from its chambers, which can only form in open air, confirm the origin.

End with a variation. These caverns on General Carrera Lake in Patagonia are cut in marble, a metamorphic rock formed from limestone about 300 million years old, and were sculpted over roughly 6,000 years by wave action of mineral-rich glacial meltwater. The swirling wall patterns record the path of the lake water.