It is 350 million years ago in a period known as the Carboniferous era and our landmass has moved northwards to the equator.

At this time, a large part of Ireland is covered by a great sea which drowned the desert plains.

During the Carboniferous the sea advanced and retreated many times in a cyclical, tidal fashion, and it took roughly 10 million years for it to spread north from Cork until it gradually covered the whole of County Fermanagh.

This sea contained great coral reefs and was rich in calcite, a material which forms the sedimentary rock limestone.

This is the core material used by many sea organisms to make their shells, including corals and algae.

Lots of these were living in our warm seas, and as they died, their shells accumulated and hardened.

Great thicknesses of debris built up, and the lower layers eventually consolidated themselves into limestone.

This makes up two-thirds of Ireland’s rock mantle today.

Therefore the major impact of this marine environment on Northern Ireland was the formation of limestone.

It’s also what comprises the gentle uplands of Fermanagh, and we can see it really well here at the Marble Arch Caves.

Calcite is a very soft material which can be dissolved by rainwater.

In fact, this soft rock is what makes up the Marble Arch Caves, which have been hollowed out by the rainwater dissolving the calcite over a period of millions of years.

To journey through these caves it to become immersed in the remains of an ancient coral sea.

This is also a silent graveyard filled with the remains of thousands and thousands of tiny sea organisms which lived and died millions of years ago.