as grain dust, a spark can trigger an explosion powerful enough to blow a concrete silo and adjacent buildings apart, usually setting the adjacent grain and building on fire. Sparks are often caused by (metal) rubbing against metal ducts; or due to static electricity produced by dust moving along the ducts when extra dry. The two main problems which will necessitate silo cleaning in bins are bridging and rat-holing. Bridging occurs when the material interlaces over the unloading mechanism at the base of the bins and blocks the flow of stored material by gravity into the unloading system. Rat-holing occurs when the material starts to adhere to the side of the bin. This will reduce the operating capacity of a bin as well as leading to cross-contamination of newer material with older material. There are a number of ways to clean a bin and many of these carry their own risks. However, since the early 1990s acoustic cleaners have become available. These are non-invasive, have minimum risk, and can offer a very cost-effective way to keep a small particle bin clean. Notable silos Henninger Turm, Frankfurt, Germany, before demolition in 2013, had an observation deck and 2 revolving restaurants, height: 120 metres Swissmill Tower, Zürich, Switzerland, height: 118 metres, the world highest silo in activity. Schapfen-Mill-Tower, Ulm, Germany, height: 115 metres Silo Tower Basel, Basel, Switzerland, has an observation deck, height: 52 metres Quaker Square, Akron, Ohio, United States, is a former set of tower silos that is now a hotel, restaurants and shops Dagon, Haifa, Israel - transformed into a museum of agriculture, a prominent local feature. Silo art "Silo art" is a recent and distinctly Australian art movement which began in Northam, Western Australia in 2015. Silos are usually decorated with huge mural-type paintings with many themes. The Australian Silo Art Trail shows 35 examples of silo art , with numbers growing. there is an ongoing legal battle over the use of the term "silo art trail", with the Yarriambiack Shire Council in Victoria attempting to trademark it in 2017, with GrainCorp, Australia's largest grains handler, opposing the move, saying that the term should be usable by many councils. Old water towers have also been decorated in many regional centres. In Melbourne, a huge painting of New Zealand Prime Minister Jacinda Ardern embracing a Muslim woman, an iconic image beamed around the world after the 2019 Christchurch mosque