Tubeless tyres are bicycle tyres run without an inner tube. The tyre seals directly against the rim, with a small volume of liquid sealant inside the casing that closes any small punctures automatically as the wheel rotates. The system has become the standard for serious gravel, mountain-bike and touring use over the past decade because of two things: the dramatically reduced puncture rate (the sealant closes most thorn or sharp-stone punctures within seconds), and the option of running lower tyre pressures, which improves both comfort and grip on rough surfaces.
On a Norwegian gravel-cycling trip, the tubeless system is genuinely useful. The mountain service roads on the Mjølkevegen and the Peer Gynt road carry occasional sharp gravel and embedded glass; on a tubed bike, the day's ride frequently includes one or two roadside punctures and the corresponding tube changes. On a tubeless bike, the same surface is essentially trouble-free for a week of riding, with the sealant visibly weeping from any small holes and resealing within the rotation of a wheel.
Most modern rental bikes in Norway are now set up tubeless as standard. Travelers bringing their own bike should check sealant levels before the trip - the liquid evaporates over months and needs topping up every six to twelve weeks of use. A small bottle of sealant in the saddle bag is reasonable insurance for a week-long trip. The full repair kit (CO2 inflator, tubeless plugs, a spare tube as last-resort backup) is light enough to carry without notice.