We finally got round to starting what will be a lengthy process of testing and analysis of what exactly lurks down the borehole - and the main thing to report so far is that the temperature down there is even higher than we'd dared to hope.

We had a lot of interest both in the region and from national press (Independent, BBC Radio 5 Live) about the notion of a new "Mine on Tyne", in the very same area where our geothermal borehole was drilled: in the heart of the 24-acre 'Science Central' site, which will be built-out over the next decade or two to become both an exemplar of a modern sustainable built environment and the place where the University's sustainability research is headquartered.

Well that unusual red sandstone that we met about one mile down persisted - all the way to 1800m in fact.  After that the rock type changed again, and we seemed to be in an unusual mixture of strata which we suppose are some of the oldest beds from the Carboniferous period. These unusual strata proved to be very hard, however, so drilling rates reduced considerably. Once it was clear that we weren't going to go back into the enticing red sandstones anytime soon, and when we got to the point where our coffers contained only small change, we decided to declare an end to deepening the borehole.  This was at a depth of 1821m - which gives us the deepest borehole ever drilled on Tyneside, and in fact the deepest direct heat use borehole in the UK (it's a wee bit deeper than the Southampton Borehole).

The delicate side-tracking operation was handled brilliantly by the team from Geometric Cofor, and we've been drilling at full speed again for two days. We're presently at 1675m, and hope to reach our final depth early next week, all being well.

In the meantime, at 9pm on Thursday 7th of July our drill-bit finally passed the one-mile deep horizon. The stone at one mile down - so our true "milestone" - is a red sandstone. That might not strike you as very odd - lots of sandstones are red where we see them at surface. Think of the beautiful sandstone in the cliffs below the Whin Sill at Bamburgh Castle, for instance: the same red sandstone out of which most of the castle itself is constructed. Normally, though, when we drill into the same sandstones at depth they are grey. This is because the iron which the contain is in the form ("ferrous") which it adopts in the absence of oxygen. Where sandstones are exposed to the air, they often change to a red colour (the colour of "ferric" iron).

We were overwhelmed with the national (and then international) press response to our project when we decided to open the site to journalists on Monday 27th June. We knew there was plenty of interest, of course, after the terrific response to the first press day we held back in February, when the first rig was on site.

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!

Well, we always knew there was a fair chance we'd hit another leaf of the Whin Sill, and so it has proved. Drilling became very difficult again, and by early afternoon on Saturday 18th June, it was obvious that the drill bit was really struggling.

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!

Having won free of the Whin Sill and made brisk progress to 914m, the drill bit encountered very hard rock again and progress slowed to a crawl. At first, the drillers wondered if we'd hit a second leaf of the Whin Sill. Looking at my earlier predictions, this would have been a real surprise as, if we hit a second leaf at all, it shouldn't be until about 1200m.

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.

At 7 a.m. this morning the drill-bit reached 914m: a prosaic number in metric units, but a magical one in feet, as it's exactly 3,000', or as deep as the loftiest peaks in the Lake District are high! Imagine the mountain of Skiddaw in the Lake District, towering above the town of Keswick: our borehole is now as deep as that mountain is high, and by the time we are done, hopefully, the borehole will be twice as deep as that!

Of all the challenges we anticipated we might encounter in drilling to great depth below Newcastle, three stood out: the risk of hitting old mineworkings; the possibility of encountering caves or large fissures in the many limestones below about 500m; and the likelihood of very slow drilling in the Whin Sill.

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.

Drilling has been proceeding briskly, at more than 100m per day, and as of noon today the borehole was already at a depth of 650m - or 2,132 feet, if you prefer old units. If you do, you are in good company, as so do our current drilling contractors, Geometric Drilling.

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!).