0:00:36 > 0:00:39This is Inis Mor, or traditionally, just Aran,
0:00:39 > 0:00:42the biggest of the three Aran Islands.
0:00:47 > 0:00:51The people only live on the north-east side of the island,
0:00:51 > 0:00:55their homes stretching like a long street from end to end.
0:00:55 > 0:00:58To the south-west lies an expanse of bare rock,
0:00:58 > 0:01:02punctuated here and there by a resilient blade of grass.
0:01:02 > 0:01:05And yet people DID once live here, on the other side of the island.
0:01:10 > 0:01:14Perched on top of dramatic 300ft cliffs is Dun Aonghusa,
0:01:14 > 0:01:19acknowledged to be one of the finest prehistoric monuments in Western Europe.
0:01:24 > 0:01:26But not for me the well-worn tourist trail.
0:01:26 > 0:01:32No, I'm off in search of another ancient fort on a cliff edge.
0:01:32 > 0:01:35Dun Duchathair...the Black Fort.
0:01:35 > 0:01:40This landscape of booming ocean and jagged rock,
0:01:40 > 0:01:47it all suggests power and strength and nothing says it more clearly than this massive curving wall.
0:01:47 > 0:01:52The Victorians rebuilt this thing but they were working from a known footprint,
0:01:52 > 0:01:56and they represented the known scale of the thing in their own hands.
0:01:56 > 0:01:59It looks like the work of giants but it's not, it's the work of people
0:01:59 > 0:02:03thousands of years ago and the scale of it takes your breath away.
0:02:05 > 0:02:09But why go to all this Herculean effort
0:02:09 > 0:02:13to cut off the end of this barren promontory?
0:02:13 > 0:02:17I've come here to meet a man who can explain what's going on.
0:02:17 > 0:02:22For some years, geologist Michael Williams has also been trying
0:02:22 > 0:02:26to solve the conundrum of the strange location of the Black Fort.
0:02:26 > 0:02:32Did the habitation of the interior actually reach all the way to the cliff edge?
0:02:32 > 0:02:36I think it certainly did because here we are in the central part
0:02:36 > 0:02:37of the promontory
0:02:37 > 0:02:41and a very exposed part of the promontory and yet here we have a constructed,
0:02:41 > 0:02:45human-constructed wall, you can see it quite clearly, the leading edge here.
0:02:45 > 0:02:48You can see courses of it there. Exactly.
0:02:48 > 0:02:53But the habitation is not restricted simply to the inside wall of that so-called promontory fort.
0:02:53 > 0:02:56It actually exists here, out in the exposed promontory.
0:02:56 > 0:03:00But what about these huge boulders, they're not part of the fort?
0:03:00 > 0:03:04People might find this hard to believe, but these are actually washed up
0:03:04 > 0:03:07on top of this promontory 65 feet above sea level,
0:03:07 > 0:03:11by the giant waves that affect these islands on a regular basis.
0:03:11 > 0:03:15So why would you go to all that effort to block off this end
0:03:15 > 0:03:19of the promontory when it's being pounded by these things?
0:03:19 > 0:03:23The mistake we're making is looking at this fort in the context of today,
0:03:23 > 0:03:29whereas we should be looking at it in the context of 2,500 years ago when it may have been built in the first place.
0:03:29 > 0:03:34If we make a sort of rough estimate of the rates of erosion that produce this kind of debris
0:03:34 > 0:03:38and say that it comes to about 0.4 metres a year.
0:03:38 > 0:03:42Extrapolate that for 2,500 years, let's say, and we're looking at land
0:03:42 > 0:03:46extending from here, a kilometre out into the Atlantic Ocean.
0:03:49 > 0:03:53So what's now known as Inis Mor, The Big Island,
0:03:53 > 0:03:572,500 years ago, was a heck of a lot bigger.
0:03:59 > 0:04:01So we're not looking at a promontory fort at all.
0:04:01 > 0:04:05We're talking about the remnants of a massive circular fort.
0:04:05 > 0:04:09So we should be enclosed now by the continuation of that massive wall.
0:04:14 > 0:04:17But what makes absolute sense is that any seas that can fling
0:04:17 > 0:04:21five tonne boulders far inland could eat a hilltop fort for breakfast.
0:04:24 > 0:04:30This destructive energy is borne out as we pass over Inis Meain and Inis Oirr,
0:04:30 > 0:04:32the other two Aran Islands.
0:04:32 > 0:04:35It's as though they've been cut in half with a blunt saw.
0:04:35 > 0:04:38MUSIC: "Theme Tune to Father Ted" by The Divine Comedy
0:04:38 > 0:04:42At the most easterly end of Inis Oirr, the rusting hulk of a cargo ship, the Plassey.
0:04:42 > 0:04:48But this ship is special, immortalised in the title sequence of the comedy series, Father Ted.
0:04:48 > 0:04:51But, no time to dawdle. We too must...
0:04:51 > 0:04:54IN IRISH ACCENT: "Go on, go on, go on."