But partly because we'd had more coverage than we ever dreamt of back then, we really thought there would be limited interest this time - how wrong we were! It was brilliant dealing with the press - I now really admire the spines of steel the journalists have, to do live broadcasts to news programmes every day: one day was nerve-wracking enough for me!

Laura identified Whin Sill cuttings coming back up the borehole at 1060m depth, so it was time to pull back and get a fresh bit on the job. Now we are down at more than a kilometre, it is a long job pulling all the rods out and inserting them all again with a fresh drill bit on the end. However, drilling was underway again before dawn, and steady progress of about 2.5 m per hour has since been made through this new, lower leaf of the Whin Sill.

It's a mixed blessing for us. On the one hand, the Whin Sill has a very low ability to conduct heat, so it can serve as a "duvet", trapping useful heat deeper below ground. In the long run, that's great; in the short run, it's a right pain to drill!

Laura Armstrong quickly inspected the cuttings (pictured below) and found that it wasn't whinstone at all, but gray recrystallised limestone. When drops of acid were placed on it, it fizzed like crazy. We interpret this to be the Scar Limestone - a bed which forms prominent cliffs up in the Pennines.

When we've drilled into it in Weardale it hasn't been particularly hard - so what explains the slow progress? Well, it's possible that the recrystallisation process in the Newcastle area toughened it up more than usual. Also, in some places around Alston the Scar Limestone is rich in nodules of chert - essentially the same hard material as flint.

L to R - Jon Busby (BGS), Michael Feliks (DECC), Paul Younger (N'cl Univ) Dave Millward (BGS) and Karl Ward (N'cl Univ)

Well, we managed to seal off the thirsty fractures associated with the old coal workings, and the limestones have been mercifully free of large fissures thus far - and the deeper we go the less likely they are to be fissured. So the Great Whin Sill was beginning to stand alone as a major threat to progress.

This isn't some eccentricity on their part - it's actually down to the fact that the global oil and gas industry (in which Geometric normally work) still works in the old-fashioned units favoured by the United States. For some reason the Americans call them "English Units", though when I lived and trained there I always pointed out that English scientists don't use such unwieldy, antiquated units anymore - and that even if we did, we'd have to insist on calling them "Imperial Units" (not a popular tag in a former colony!).

We had always anticipated a gap of a few weeks between completion of the first phase of the borehole and the start of the main phase. When Drilcorp completed the first section of the borehole to 245.5m ahead of schedule, that made it look like we'd have four or five weeks to wait, while we finalised contractual arrangements with the specialist sub-contractors for the deeper reaches of the borehole. That job turned out to be a lot more challenging than we'd bargained for.

In view of the fractures at 161m, this actually meant filling the borehole back to that level with grout, which we then had to drill out. We completed all of that by Thursday 24th March, so the first drilling rig was then able to leave site.

Then on Monday 28th, we had a red letter day - the Minister for Energy and Climate Change, the Rt Hon Chris Huhne MP, came to visit the borehole project. As Minister Huhne's Department are one of the main funders of the project, we were delighted he was able to visit us.

What an irony, though, after all the frantic activity, that his visit happened to fall in the only time in 6 weeks that there hasn't been a drilling rig on site! However, the Minister was very understanding, and indulged in the time-honoured practice of looking at the concrete cover and imagining the rocks and pipework down below.

Apart from a few wee spots where the slightly deeper Marshall Green and Victoria seams were worked (e.g. around Widdrington and Sacriston respectively), the Brockwell Seam is the deepest (and oldest) coal seam to have been routinely worked in the Great Northern Coalfield.

Bang on cue, the drill bit suddenly speeded up, and the rig cut through the Brockwell Seam like a knife through butter. That was great, because if it had cut like a knife through thin air, we'd have had even greater problems maintaining flushing of the borehole with water.

Q: "Tim Marlton said: Sounds like a very exciting project. If heat is removed from the earth's core, the core will cool down a little faster than its normal rate - are there thought to be any side effects of this?"

A: Good question. Actually it's not heat conducted from the Earth's core (or mantle) we're seeking here, but heat generated high up in the Earth's crust be slow, natural, harmless, radioactivity. Although it is possible to "overdraw" this locally, most boreholes will function well for several decades before they need to be "rested". In the meantime, replacement boreholes can be drilled nearby in good time. The original boreholes can be brought back on line later.

Now if you believe the old records, the Harvey wasn't actually accessed from the North Elswick Colliery shaft nearby. However, as accurate mine plans were only kept routinely after about 1875, you can't be sure that you won't hit old workings, especially if the seam is thicker than about one metre. (Even 0.5m seams were sometimes worked - spare a thought for the poor lads that had to do that!).