A team of celebrities will start their Sport Relief challenge across the Namib Desert in Africa today.
Each of them are travelling by foot, bike and skis, yes skis (!), across 100 miles of desert.
The skis can be used on sand instead though.
The celebrities will have to deal with 35 degree Celsius heat as they walk in the baking hot sun across some of the highest sand dunes on earth.
The Namib desert located in Namibia in Africa is known as the oldest desert in the world.
The celebs are taking on the challenge to highlight mental health and the ways in which people can try to stay mentally strong when facing something really difficult.
One in four people will be affected by a mental health problem in their lifetime, and each of celebs taking part say they are passionate about speaking out about the issue.
Before setting off, Karim shared his thoughts on social media saying: "It's a great cause, we're all very excited, but we're all very nervous because it's going to be really, really tough.
"We've got a goal set, we're gonna smash it, spirits are up and we're hoping to raise as much funds as we can and of course raise awareness for mental health which we are all very passionate about."
An hour-long documentary will show the team's highs and lows during the journey on BBC One from 9 March.
Dr Zoe Williams, who has been offering support to the celebrities over the past couple of months, says: "Their main physical challenges now will be dehydration and heat exhaustion. They will need to take really good care of their feet because sand combined with heat is a menace on the feet."
Sport Relief is back on Friday 13 March.