Tasaka, YSchneider, Tobias MMullin, T2015-12-072015-12-072015-12-07201010.1103/PhysRevLett.105.174502https://infoscience.epfl.ch/handle/20.500.14299/121513The results of an experimental investigation into the threshold boundary between laminar and disordered pipe flow are presented. Complex features have been uncovered using a highly refined experimental approach where an intermediate periodic state forms an integral part of the transition sequence. In accord with the suggestions produced by a numerical investigation, the boundary is found to be folded with a complicated structure. This raises important questions about accepted definitions of threshold amplitudes in this long-standing problem. © 2010 The American Physical Society.text::journal::journal article::research article