It is April and the smell of fire is in the air again. We have just driven 40-odd kilometers from Pai to Pangmapha District over a dry and dusty pass, where backhoes are still clearing the debris of last year's storms. Heat lies heavy on the day, the light strangely yellow through a haze of humidity and smoke. But nearing the entrance to Tham Lod the temperature drops sharply, a welcome blast of stone-cooled air emanating from a yawning hole in the cliff. Our guide - appropriately named Yen - stops briefly to light a paraffin lantern, then leads us into the inkiness of the cave's interior.

Tham Lod (Through Cave) is but one of a vast number of cavern systems in Pangmapha, carved by wind and water into ubiquitous limestone cliffs that characterize the region. Though Tham Lod is striking in its sheer magnitude and superb lime carbonate formations alone, the cave is also known for its archaeological significance. An astounding amount of evidence indicating early human habitation has been collected here, including a prehistoric skeleton dating back 20,000 years. The cave is also home to ancient phee man log coffins : carved wooden tombs that have also been found in karst areas west of Bangkok and locations across Southeast Asia, including Southern China, Sumatra and the Philippines. Nowhere, however, have they been discovered in such high numbers as in Pangmapha, where over 80 such locations have been recorded.

Just who the Log Coffin people were remains an enigma. Pangmapha's local Shan believe that they were tall spirits who wandered the caves and used the tombs as their resting places, and to them these cavecrypts are known as Tham Pee Man, translating to 'large spirit caves'. The coffins were chiseled from massive trunks of teak, split in half and hollowed out, so that each had a base and lid, many bearing carved head posts in over 50 different styles. Along with occasional fragments of bone, many coffins were found to contain remnants of clay pottery and beads from foreign lands, somehow traded with this distant culture. Carbon dating shows that the coffins burials were performed over hundreds of years, yet most caves have fewer than ten coffins, leading archaeologists to believe that the Log Coffin people belonged to the upper echelons of a stratified society and were buried along with precious possessions for use in an elaborate afterlife. Dating from around 1700 to 1500 years ago, most are in an excellent state of preservation for their age and some remain elevated on sturdy teak poles strengthened by crossbeams.

Inside the cave it is deathly quiet but for the faint flow of water, sounds of the outside world sealed off behind thick walls of limestone. Sloping downwards from the mouth, an entrance passage opens into a cavernous inner chamber dotted with stalactites and stalagmites, formed over thousands of years of slowly dripping water. The lantern's glow bounces off the calcite pillars, revealing infinite pin pricks of glittering mineral deposits and I imagine the cave as it may once have been, lit by fiery torches and echoing with ancestral voices. To our prehistoric brethren Tham Lod must have been seemed a gift from the gods - or the spirits - a vast place of shelter coupled with a perennial water source, the Lang River, which flows along the base of the cave system. By now my eyes have adjusted to the darkness and crossing a bridge over the water I can make out hundreds of massive carp resting on the stream's pebbled floor, moving sluggishly against the current. Protected by locals and fattened on bags of fish food peddled to tourists, the carp continue to grow in size and number as the years pass. Up at Cave Lodge later, I talk to Australian owner John Spies, a Pangmapha veteran who has been exploring the area since the 70s and is a wealth of information on its ecology, history and archaeology. He tells me that the food pellets are only a supplement to the carp's natural diet - baby swifts that fall from the cavern ceiling.

In fact, Tham Lod is a living, breathing entity, a fragile ecosystem encased in solid stone. It is filled with a bizarre menagerie of cave-adapted creatures - agile racer snakes that slither overhead to feed on bats and swifts, micro-snails that survive on bacteria coating the rock walls, troglodyte spiders and blind fish. Thousands of bats from several different species cover the cave ceiling by day, and their exodus at dusk makes way for Fork-Tailed and Pacific swifts that shelter there by night. In a mind-boggling feat of twilight orchestration, over 300,000 birds enter the cave at lightning speed, breaking off in small swirls and using complex echolocation techniques to stay on course and avoid collisions.

Though the Log Coffin caves are Pangmapha's chief attraction, the area offers a kaleidoscope of nature and adventure activities beyond these eerie burial sites. As yet untainted by the urban sprawl that is inching across Pai, the region is still a jungle wilderness of remote trails, traditional hill tribes and prolific flora and fauna. Cave Lodge, Mae Hong Son's oldest guesthouse, offers an adrenalin-inducing selection of kayaking, caving and trekking tours. The surrounding rain forest provides sanctuary to abundant reptiles, small mammals and avian species, along with enough creepy-crawlies to fill a lifetime of twisted entomological fantasies. Best of all, says Spies, the long-absent gibbons are back. Scared off for years by the human intrusion into their primordial habitat, they are slowly returning to their original home, their morning calls mingling with the muffled clang of cowbells.