Almost as soon as RFK Stadium opened in 1961, it became clear that the stadium was a dud. Football fans complained that the low seating made it difficult to see the entire field. Baseball fans complained that they had to twist in their seats to see the action at home plate. By trying to accommodate two sports, the stadium failed at both. All dozen of the combination football-baseball stadiums built in the U.S. since then have garnered similar complaints. The only one still in use as a dual-sport venue, the Oakland Coliseum, consistently ranks among fans as one of the worst sports arenas on the continent. Yet despite this dismal record, some architects are considering multi-sport venues again. PopSci asked Greg Sherlock, an architect at Kansas City–based Populous, the world's leading stadium-design firm, to give us a look at the future. His concept: a truly transformable stadium, whose modular parts snap together like Legos in custom configurations.