If testing, torturous steep climbs are what you're after, here are ten that will get your leg muscles straining just reading about them. The upside to struggling to the top of these summits though is of course the magical views that await you (if you're not too tired to notice).
From short climbs in Cumbria to absolutely epic climbs in Hawaii, none of these are easy but some certainly require less training than others.
Training plans to help you get to the top of most of these routes are available on Climb By Bike.
Often defined as the hardest climb in the world, Le Mauna Kea begins at sea level in Hilo and gradually elevates up over a 68.6 kilometre stretch to the finishing point at 4191 metres.
Up until the visitor centre the trail is somewhat doable, but from here until the summit you're in for pure torturous climbing.
Traversing through the Alps in Abruzzo, Italy, the climb from Berbenno to Caldenno stretches for 15 kilometres gaining a total of 2015 metres in height.
Starting at 390 metres the route gets gradually steeper and steeper until you arrive at the final kilometre where the climbing battle really begins.
Beginning in Kodari, Nepal and continuing for 100 kilometres into the Himalaya, the Tong La Pass is one of the longest uphill cycle routes in the world.
The climb starts at 1510 metres, gradually reaching 5130 metres at its highest point. Passing from Nepal into Tibet, the panoramic views are simply stunning.
Peru's most difficult climb, the Casma - Huaraz route gains 4140 metres over a course of 112.3 kilometres. The trail begins in Casma, Ancash at just 85 metres in altitude before the long, tiresome ascent begins.
Mountain bikers often struggle to complete this route so to make sure you are fully prepared - have a read of a recommended training plan here.
Starting in Les Clapiers, the Col du Jandri cycle route climbs 2458 metres into the Rhone-Alpes over a 26.9 kilometre stretch.
The struggle to the top is worth it for the hypnotic views over the Glacier Mont de Lans you will be awarded with.
Situated in the beautiful surroundings of Amazonas in the Venezuelan Cordillera de los Andes, the Pico Aguila mountain biking route is an enduring 66.17 kilometres of ascent.
Starting in Valera at 503 metres, the epic ride finishes at an impressive height of 3963 metres offering incredible views across the Cordillera.
A ride through the Sierra Nevada for the most serious of mountain bikers, the Pico de Veleta is a 40 kilometre ascent that begins with a stint on Europe's highest road.
Starting at an altitude of just 700 metres the road quickly inclines and continues to do so throughout the rest of the ride, with the gradient never dropping below 6% after a teasingly short descent very early on.
The second half of the trail is extremely gravelly, making riding conditions tougher still. It's a challenge, but being so close to home it's also a great one to get ticked off the bucket list.
The beautiful mountain region of Bulgaria, Rila, is home to the climb that ranks twentieth in the world's difficulty list. Starting in Rila in Pazardzhik, the route continues for 14.5 kilometres from a height of 800 metres to 2500 metres at the top point in Pastra.
All the way up you are awarded with stunning mountainous views over a rarely explored country, completely justifying the testing ascent.
One of the toughest climbs in the UK, the Hardknott Pass in Erksdale, Cumbria gains 298 metres in height over a 2.25 kilometre-long climb. Battling with switchbacks and sharp corners, this climb is anything but easy despite the short distance that it covers.
The climb begins at the warning sign at Jubilee Bridge and takes approximately fifteen minutes to reach the top.
Also known as La Gamonal, the Alto de L'Angliru is in Asturias, Northern Spain and makes for a relatively short but incredibly steep climb.
A brilliant route for professional road bicycle training, the trail starts at 322 metres in height and gains a further 1248 kilometres over the 13.47 kilometre route.
At its steepest point, Cueña les Cabres, the gradient is 23.6% with the average gradient along the trail being 9.9%.
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