K24 Ultra-Trail 100km — 6km Underground Through Slovenia's Peca Mine and 5,420m D+ in the Koroška Alps

The Podzemlje Pece (Peca Underworld) passage is the K24's global differentiator: 6 to 6.5km of the 100km route runs underground through an abandoned zinc and lead mine that operated for more than 360 years above Črna na Koroškem. Headlamps are mandatory; helmets available. The race describes this as 'a unique running event on a global scale,' and nothing comparable exists in the DTR database. The mine tunnels introduce conditions no other trail race in this database features: total darkness outside the headlamp beam, consistent underground temperatures regardless of surface weather, and a completely distinct psychological environment from the alpine ridge running above ground. Outside the mine section, the race is a technically demanding 100km mountain ultra with 5,420m D+, a rank-93/~390 opening ascent of 1,051m at 19.5% average from km 9.3, and a 13.94% DW average uphill (rank 119/~390 — top 30% steepest).

The K24 opens immediately into extreme terrain. The biggest single ascent (1,051m / 19.5% avg / 5.4km, rank 93/~390 — top 24% by size) begins at km 9.3; within 15km, runners have gained over 1,051m at near-constant extreme gradient. Descent 2 (841m / -15.4% avg / 5.5km) begins at km 15.5, directly after the top climb — creating a back-to-back that delivers nearly 1,900m of vertical change in the first 21km. After Descent 3 (km 31.2, 554m) and Climb 2 (km 36, 758m / 12.2% avg), the mine tunnel passage through the Podzemlje Pece arrives mid-race. The course then delivers a late back-to-back: Climb 3 (623m / 16.2% avg) at km 75 feeds immediately into the biggest descent (1,090m / -11.5% avg / 9.5km, rank 180/~390) at km 78.7 — over 1,700m of vertical change at 75–88% of race distance. The 19% flat terrain (~19km, rank 105/~390 — top 27% most mountainous) is distributed between these major blocks.

The K24 suits mountain ultra runners who can handle extreme early-race gradient demand and genuinely unusual terrain. The 19.5% average opening climb (1,051m, km 9.3) requires specific training: opening long runs on maximum grade before the body has warmed up. The mine tunnel section requires comfort in enclosed, dark environments — headlamp familiarity matters. The 13.94% DW avg uphill (rank 119/~390 — top 30% steepest) warrants specific sustained-gradient training above 15% for 30+ minute blocks. The late back-to-back (Climb 3 at km 75 into Descent 1 at km 78.7, rank 180/~390) demands late-race vertical conditioning. With 19% flat terrain (rank 105/~390), the K24 is not a relentless grade machine, but its gradient blocks arrive at the worst possible moments: immediately from the start and at maximum accumulated fatigue late in the race. Poles are recommended.

At EUR 120 (AUTO EUR 1.20/km — last confirmed Jan–May 2026; check k24trail.si for current pricing), the K24 offers strong value for a UTMB Index 100km with a globally unique feature. The ~95-person finisher field [LOW CONFIDENCE] makes it one of the most intimate 100km events in Europe — part of the Alpe Adria Trail Cup and UTMB-indexed, yet with a small, specialist field. The mine tunnel passage is not replicated anywhere in the DTR database. The course is demanding (5,420m D+, rank 119/~390 avg uphill, extreme opening and late gradient blocks) but the 19% flat terrain (rank 105/~390) makes it more accessible than ultra-high-gradient events like Scenic Trail 120km (~9.5% flat) or MF100 (~9.7% flat). The race is not sold out. The 2026 edition runs August 7.

Ljubljana Airport (LJU) is the primary airport at 108 minutes to Črna na Koroškem — not IS FAR by the DTR 180-minute threshold. Klagenfurt (KLU, Austria) and Trieste (TRS, Italy) are also viable depending on routing. Public transport is limited — bus or train to Slovenj Gradec or Ravne na Koroškem, then taxi; a car is strongly recommended. Accommodation is limited in Črna na Koroškem; Slovenj Gradec (~30km south) is a better base with stronger hotel supply. Book early for the August 7–9 race weekend — more than 500 competitors across all distances. The 100km starts August 7, 2026; the 50km on August 8. Check k24trail.si for shuttle arrangements.