Palest gleam in the deepest dark

Among the many wonders of the world I shall never see is the Great Stalactite of Pol-an-Ionain, 6.5 metres long, suspended at the centre of a great cavern in the limestone underworld of the Burren. To reach it needs an interminable crawl through a tight, 150metre-long passage, described by cavers themselves as "knee-wrecking" and "miserable", not to mention stoic indifference to any twinge of claustrophobia.

For about a decade, the owners of the land above the cave's vaulted ceiling have been nurturing plans to "open it up" as a tourist attraction like the very successful one at Ailwee. Cavers and scientists alike have been resisting the idea in some horror, and a planning refusal is on appeal.

The huge, sparkling stalactite, the biggest so far in Europe - if not the world - is the product of a particular chemistry of seeping acid water, lime and carbon dioxide that has continued for thousands of years. Atmospheric change, vibrations and vandalism would put it at risk.

Aside from that, there should be wonderful things in nature that are only accessible to people by huge personal effort - if at all. I prize the knowledge that this massively beautiful structure hangs in the dark and silence of its pre-Ice Age chamber, inaccessible by helicopter, cable-car, submarine, allterrain vehicle or tourist bus - if you want to see it, you have to crawl. Cave access and conservation are the stuff of a special symposium in Enniskillen, Co Fermanagh, this weekend, presented by the Speleological Union of Ireland. It is specially concerned with karst, the geological phenomenon that riddles this island with interesting holes.

Just as drumlin and esker were words borrowed from Irish to describe special features of the post-glacial landscape, "karst" has emerged from Serbo-Croat to label the eroded limestone uplands in the former Yugoslavia - and all similar regions, such as the Burren and the limestone hills of Sligo and Fermanagh.

Pure limestone can be dissolved into dramatic rock-sculptures by the action of weathering and naturally acid water. Just before the Ice Age, much of central Ireland was bare limestone, worn down from a vast block uplifted from former oceans and fretted into pinnacles, gorges and caves. The glaciers ploughed down much of this jagged high relief and then, in their retreat, smothered the fractured lowland karsts with a thick layer of boulder clay, sand and gravel.

Only lately have Ireland's geologists woken up to the extent to which rainwater, following every crack and fissure in the rock, has turned much of the island's limestone raft, especially the purer rock west of the Shannon, into something resembling a vast, stone sponge. The implications are urgent on a fast-developing island, as pollution sweeps through groundwater aquifer channels, as motorways cut into bedrock, and foundations are drilled ever deeper for big new construction.

Trinity College geologists have joined with the Geological Survey to build a computer database that amasses every local clue to karstification. At the same time, the GSI is selecting "the very best of Irish karst" for conservation as Natural Heritage Areas. In Enniskillen today, Dr Matthew Parkes will report on its progress.

Among the priorities in the Geological Heritage Programme are the turloughs, the vanishing lakes that fill and empty through the action of underground fissures. More difficult is to pick bigger karstic systems which show the effects of their plumbing and rock-sculpture over a wide area: they need conserving against a variety of threats, from waste-tips and fertiliser run-off to dead-cow disposal.

None of this is likely to add to the great showcaves of Ireland, such as Marble Arch, on the Fermanagh side of Cuilcagh Mountain, Ailwee in the Burren, Crag Cave at Castleisland in Kerry and Mitchelstown in Tipperary.

But show-caves, with their spotlights, walkways and teashops, are a million miles in spirit from the adventure-sport-cum-earth-science of speleology. This typically finds its satisfactions in flood-prone passages beneath the Burren and Sligo-Fermanagh hills where, to quote the caving writer Philip Chapman, "sinuous underground canyons, with their glistening water-sculpted walls echoing the sound of rushing water, represent the nearest thing to paradise on earth".

Access to such systems, and the thousands of kilometres that remain to be discovered and explored, is through pot-holes and other, sometimes minuscule, crevices, most of which are on somebody's land. Like hill-walkers, cavers are beginning to meet the rough edge of resentment among farmers marginalised from the new upbeat mood of national affluence.

Should landowners charge admission to the secret labyrinth beneath their stony pastures? Even at a fiver a time, they are unlikely to get rich from cavers, a thinly scattered weekend tribe. And payment carries implications for management and, above all, "liability".

In Enniskillen tomorrow, the management of access to the countryside will be discussed from two significantly different perspectives, North and South. A speaker from CAAN, the Countryside Access and Activities Network of Northern Ireland, will describe how, with state funding and encouragement, it co-ordinates the pastimes of the soaring number of "user groups", in consultation with farming organisations and other rural landowners.

A speaker from Keep Ireland Open, on the other hand, will describe the struggles and confrontations that are bound to increase in the Republic, where no strategic overview or agency yet exists to help sort out conflicts in access to the countryside.