Tour de France leader Bradley Wiggins faced four formidable and legendary obstacles in his bid to hold on to the yellow jersey for a 10th day.
Wiggins began the 197-kilometre 16th stage form Pau to Bagneres-de-Luchon with an advantage of two minutes five seconds over Team Sky colleague Chris Froome, with Vincenzo Nibali (Liquigas-Cannondale) the nearest hostile rival, 2mins 23secs behind.
Only three riders from rival teams were within five minutes of Wiggins, who was wearing the maillot jaune for a ninth day, with defending champion Cadel Evans (BMC Racing) 3:19 adrift and Jurgen van den Broeck (Lotto-Belisol) 4:48 behind, but the route was the first of two challenging days in the Pyrenees.
The mountains are where the Tour is invariably won and today's stage featured the fabled climbs of the Col d'Aubisque, Col du Tourmalet, Col d'Aspin and Col de Peyresourde, the summit of which came 15.5km from the finish in a valley.
The 17th stage, finishing at the summit of Peyragudes, was another testing day which awaits, while should Wiggins enter Saturday's 53.5km time-trial to Chartres even facing a small deficit, he would be fancied to be the Tour's first British winner.
Wiggins is imperious against the clock and won the stage nine time-trial to Besancon.
The 32-year-old triple Olympic champion must first negotiate the Pyrenees safely.
An early escape featuring more than riders - the best placed overall being Spain's Egoi Martinez (Euskaltel-Euskadi), who began the day more than 18 minutes behind the Briton.
Briton Steve Cummings and his BMC Racing team-mate George Hincapie were also in the escape, suggesting Evans was eager for support up the road entering the mountains.