A visit to West Cornwall feels a bit like a trip to the edge of the universe: windswept moorland, sparsely popu­lated, with the huge blue sea down at the end of the fields, where the grass gives way to the cliffs. So it feels entirely appropriate that when you get to St Ives, the only place to go is into the rock face; and that’s what they’ve done at the Tate’s furthest outpost, with a huge underground gallery excavated into the hillside.