Graham, this is turning into a very interesting post. Do you know what evidence there is for a hydrothermal origin for Pen Park Hole? The UK has many examples of post-mineralisation caverns, which could have formed at virtually any time in the past 200 million years, but we're talking about something different here aren't we, which are caverns forming at the same time as late-stage hydrothermal mineralisation due to uplift following the closure of the Rheic Ocean in late Carboniferous - early Permian times.