Robert Lowell wrote five books of unrhymed "American sonnets", including his Pulitzer Prize-winning volume "The Dolphin" (1973). Half-rhymed, unrhymed, and even unmetrical sonnets have been very popular since 1950; perhaps the best works in the genre are Seamus Heaney's "Glanmore Sonnets" and "Clearances," both of which use half rhymes, and Geoffrey Hill's mid-period sequence "An Apology for the Revival of Christian Architecture in England". Without a doubt, the most ambitious sonnet project of the late 20th century is Vikram Seth's "The Golden Gate" (1986), a comic celebration of life in San Francisco in the early 1980s in nearly 600 sonnets (even the acknowledgements and table of contents are sonnets). The 1990s saw something of a formalist revival, however, and several traditional sonnets have been written in the past decade, including Don Paterson's "40 Sonnets" (2015).