The capacity of access links has increased dramatically in recent times, and bottlenecks are moving deeper into the Internet core. When bottlenecks occur in a core (or AS-AS peering) link, it is possible to use additional detour paths to improve the end-to-end throughput between a pair of source and destination nodes. We propose and evaluate a new massively multipath (mPath) source routing algorithm to improve end-to-end throughput for high-volume data transfers. We demonstrate that our algorithm is practical by implementing a system that employs a set of proxies to establish one-hop detour paths between the source and destination nodes. Our algorithm can fully utilize the available access link bandwidth when good proxied paths are available, without sacrificing TCP-friendliness, and achieves throughput comparable to TCP when such paths cannot be found. For 40 percent of our test cases on PlanetLab, mPath achieved significant improvements in throughput. Among these, 50 percent achieved a throughput of more than twice that of TCP.