Rumoured to run to 120 miles, Russia is digging a vast trench stretching the full length of Ukraine’s eastern flank. An enormous feat of engineering, but does it mean they’re digging in or desperate? The BBC’s security correspondent, Frank Gardner, tells us that this line of defence, like the Maginot Line before it, is no guarantee against a Ukrainian advance. We also hear about the alleged use of cluster bombs. Human Rights Watch says it’s seen the tell-tale fragmentary patterns left by the explosions and spoken to people who have lost limbs. And Owen Matthews, author of 'Overreach', explains why President Putin invaded Ukraine, and the reasons it isn't going well for him. Plus listeners share Ukrainian songs and artwork they’ve discovered since the war began. Today’s episode is presented by Victoria Derbyshire and Vitaly Shevchenko. The producers were Arsenii Sokolov, Clare Williamson and Luke Radcliff. The technical producer was Emma Crowe. The series producer is Fiona Leach. The assistant editor is Alison Gee and the editor is Jonathan Aspinwall. Email Ukrainecast@bbc.co.uk with your questions and comments. You can also send us a message or voice note via WhatsApp, Signal or Telegram to +44 330 1239480