Recently, I was reading about the Khewra Salt Mine, and the information truly surprised me. That was when I realised that we talk so much about mountains, rivers and valleys around the world, but we hardly talk about the unique underground world — caves.

Caves are natural wonders, some explored and hundreds still unexplored. The more you read, the more you realise how little we actually know about this hidden and fascinating world.

Pakistan has far more caves than most of us imagine. Some caves have been explored, while many are still untouched. So far, documented cave systems in the country mainly include limestone karst caves, mountain caves and several vertical shafts that are still under study.

I will be exploring more of these hidden geological gems in the future. For now, I have shortlisted a few from dozens of remarkable sites, each with its own amazing and unique ecosystem, structure or formation pattern. Choosing just a few from such a long list feels almost like an injustice. However, I have tried to keep some of the rarest and most extraordinary ones at the top.

From glowing ceilings to crystal giants, caves hold secrets that feel almost unreal. Let’s step into the hidden universe of caves and discover their magic

Chemosynthetic ecosystem

Movile Cave — Romania: This cave is not beautiful in the usual way. It is not colourful either. It remained sealed from the outside world for about 5.5 million years!

There is no sunlight and very little fresh air. Yet, do you know that life still exists inside it? Yes — but not ordinary life.

There are no plants. No sunlight-based food chain. The insects and spiders living there are blind and pale. They evolved entirely in that darkness. Many of them do not exist anywhere else on Earth. They have never seen the sun — not even once.

What makes Movile Cave extremely unique is not just the darkness, but the fact that the entire ecosystem runs on chemistry instead of light. Microorganisms use chemosynthesis, deriving energy from chemical reactions involving hydrogen sulphide rather than sunlight. That is very rare on land.

The air inside is toxic for humans because of high levels of hydrogen sulphide and carbon dioxide — gases that can make a person seriously ill within minutes. No human can survive in Movile Cave without specialised breathing equipment and protective gear.

Chemosynthetic ecosystems are chemical-based ecosystems where life is supported by chemosynthesis instead of sunlight. They are among the rarest ecosystems on Earth.

Sulphur-based chemosynthetic ecosystem

Ayalon Cave: This cave is similar to Movile, yet different in several ways. It was discovered by accident in a quarry when workers unexpectedly broke into it and revealed an entirely hidden underground ecosystem.

Here again, there is no sunlight, and life depends on chemical reactions rather than photosynthesis. One of the most surprising discoveries was a blind shrimp-like crustacean. It survives with the help of bacteria that feed on sulphur compounds.

This kind of ecosystem is extremely rare and with only a few locations in the world where life depends almost entirely on chemosynthesis.

Mineral crystal cave Ochtinská

Aragonite Cave — Slovakia: Most caves around the world are limestone caves formed mainly of calcite. However, this cave is unique because of its delicate crystal formations made of aragonite, a rare crystalline form of calcium carbonate that grows in bra­nching, coral-like or bush-like shapes.

The white aragonite formations look soft and fragile, almost like frozen flowers. Only a small number of caves worldwide are known for such extensive aragonite decoration, which makes this cave scientifically and visually significant.

Subterranean rainforest ecosystem

Son Đoòng Cave — Vietnam: This cave lies beneath the Truong Son Mountains in Vietnam and contains the largest known cave passage in the world by volume. The cave is so enormous that it could fit a 40-storey building inside.

It has massive stalagmites, some nearly as tall as skyscrapers, and cave pearls that can grow to the size of baseballs.

What makes this cave even more extraordinary is not just its size, but the fact that it contains its own small rainforest ecosystem. Thousands of years ago, parts of the cave ceiling collapsed, allowing sunlight to enter. Because of this, trees and various plants began to grow, slowly forming a lush forest deep underground.

What amazed me even more is that clouds can form inside the cave due to differences in temperature and humidity. In a way, it has developed its own small weather system.

While there are other enormous caves around the world — such as Hang Én Cave, Deer Cave and Mammoth Cave — none are known to contain a fully developed subterranean rainforest ecosystem on this scale.

Hydrothermal crystal formation

Cave of the Crystals — Mexico: This cave is connected to the Naica Mine, about 300 metres below the surface in Chihuahua, Mexico.

It contains giant selenite crystals (a transparent variety of gypsum), some of the largest natural crystals ever discovered. The largest known crystal measures about 12 metres in length, 4 metres in diameter and weighs approximately 55 tonnes.

The cave is extremely hot, with air temperatures reaching up to 58°C and humidity levels between 90 and 99 percent. It is nearly impossible for humans to remain inside without special cooling suits and breathing support.

The crystals formed over hundreds of thousands of years under very specific temperature and mineral-rich water conditions. Very few places on Earth have produced crystals of this size formed in such a way.

Glow ecosystem

Waitomo Glowworm Caves — New Zealand: At first glance, this cave feels like something straight out of a fairy tale, with glowing beautifully and mesmerising everyone who enters.

So what makes it glow? It is the glowworms, actually the larval stage of a fungus gnat species called Arachnocampa luminosa. These larvae produce a blue-green light through a chemical reaction to attract prey into their sticky threads.

Glowworms exist in a few other places as well, but the concentration here creates a natural “starry roof” effect that is rare at such a scale.

Salt Cave system

Malham Cave — Israel: It is the longest salt cave in the world. Most caves are limestone, but salt caves are different. They form much faster because salt dissolves quickly in water.

This also makes them fragile and relatively temporary in geological terms. Therefore, having such an extensive salt cave system is considered rare.

Volcanic column sea cave

Fingal’s Cave — Scotland: The Gaelic name of Fingal’s Cave is An Uaimh Bhinn, meaning “the melodious cave.”

The cave has a large arched entrance opening directly to the sea. When waves enter, the sound echoes deeply, creating a natural musical and resonant effect.

It is formed entirely from hexagonally jointed basalt columns. These formations occur when slowly cooling lava contracts and cracks into regular geometric patterns, creating the distinctive hexagonal rock pillars that make the cave visually striking.

Chiropteran-dominated cave ecosystem

Deer Cave — Malaysia: This cave has one of the largest cave entrances in the world. What makes it especially remarkable is the enormous bat colony that lives inside it. Millions of bats reside there.

When they leave the cave in the evening, they form a dramatic black spiral shape in the sky — a breathtaking sight.

Bats commonly live in caves around the world, but the size of this colony and its organised flight pattern are almost unbelievable. Other notable large bat colonies include Bracken Cave.

Acid-formed cave

Lechuguilla Cave — USA: Only a few caves around the world are known to have formed primarily through sulphuric acid speleogenesis, rather than simple rainwater erosion. Lechuguilla Cave is one of the most well-documented examples.

Many of its formations developed when sulphuric acid dissolved rock from below, not just from rainwater seeping from above.

Scientists have also discovered unusual bacteria inside that show natural resistance to certain antibiotics, even though they were never exposed to modern medicine. This makes the cave scientifically important as well as geologically unique.

Eisriesenwelt Ice Cave — Austria: This cave is one of the largest ice caves in the world. What makes it rare is not just the presence of ice, as many caves contain seasonal ice, but the massive frozen formations inside a limestone mountain, including giant ice walls and frozen waterfalls.

The unusual airflow system inside the cave helps maintain low temperatures, keeping the ice preserved even when outside temperatures change.

Reed Flute Cave — China: The name “Reed Flute” comes from the reed grass growing outside the cave, traditionally used to make flutes and pipes.

The cave contains colourful limestone formations, enhanced today with artificial lighting. It has winding walkways, ponds, bridges and beautifully illuminated stalactites and stalagmites that reflect light in unusual ways.

The cave has been known and visited for more than 1,000 years.

Published in Dawn, Young World, February 28th, 2026