A Japanese timber onsen above a glass-walled AI data centre, a snow-monkey sanctuary and a restaurant & whisky bar above Vienna — heated by the servers beneath it.
Watch the film · 2:25The film shows the first concept study. Since then the project has become smaller, quieter and made of timber — the current plan is below.
Almost every watt a GPU draws ends up as heat — and most data centres simply vent it into the air. YUKI puts a small, liquid-cooled AI data centre into the half-sunk lower level of a Japanese timber onsen on the Kahlenberg, next to a forest sanctuary for Japanese snow monkeys, with a restaurant and whisky bar on top. The compute pays for the heat. The heat creates the experience.
Kahlenberg — Vienna’s classic viewpoint, high above the city and the Danube.
of the electricity an AI server consumes is turned into heat.
base-case investment with a data-centre partner — a compact project instead of a mega-resort.
View from the Kahlenberg at duskOn the edge of the Vienna Woods, 484 metres above sea level, the Kahlenberg is the viewpoint every visitor to Vienna knows: the Danube, the skyline and, on clear days, the Alps on the horizon.
You park and climb a broad stone stair straight into the ground floor. The server hall lies half sunk beneath it, three metres below the car park: through a glass band at street level you look down on the AI servers, and from the reception you stand on a glass floor above them. Above rise three storeys of timber onsen — baths, a large sauna world, rest rooms and private baths — with the restaurant and bar on top. At level 2 a forest-sauna wing and the open-air deck reach out over the slope on stilts, towards the macaque sanctuary.
Three floors of 704 m² plus the forest wing — reception with a glass floor, indoor baths, a sauna world with six saunas, tatami rest room, tea salon, private baths, treatments.
On level 3, above the treetops — around 50 seats with an omakase counter, plus a bar for about 20.
Open-air baths at level 2 around the forest wing, on stilts over the slope: about 120 m² of water at 40–42 °C, a sauna hut, facing the sanctuary.
A fenced forest enclosure with two warm springs, an animal house and a public boardwalk to an outlook.
Half sunk below the car park: one continuous hall with 60 racks, seen from above through the glass band and the glass floor. Its heat rises straight into the baths above.
Cedar and hinoki, charred-timber façade and layered eaves above the servers. The car park stays; the access lane moves in front of the house.
The house by day — stair and glass band at the car park, four floors, forest wing at the back
The whole site — house, forest wing, sanctuary and the short heat loopConcept stage · 3D model built on City of Vienna open data (terrain, aerial imagery, zoning) · Site plan as PDF (German)
The server hall and the plant fill the half-sunk lower level. A stone stair leads from the car park straight into the ground floor: reception on a glass floor above the servers, changing rooms, indoor baths and an ashiyu foot-bath lounge. Level 1 is a sauna world — a panorama sauna for 60 facing Vienna, soft, salt, steam and women-only saunas, plunge pool, Kneipp walk and rest rooms. Level 2 holds tatami rest, tea, three private baths and treatments, and opens onto the forest wing and the open-air deck. Restaurant and whisky bar on top.
3D cutaway · level 1, the sauna world
3D cutaway · level 2 with the forest wing and the rotenburo deck
Reference imagery for mood and materials (Adobe Stock) — not the final design.
Liquid-cooled GPUs return water at around 55 °C — hotter than any onsen needs. Instead of venting it through cooling towers, the warm water rises straight from the servers on the lower level into the baths above, then runs through the macaque springs and the restaurant. Only the surplus travels further, along about 280 metres of pipe to the neighbours, before the water flows back to the servers.
1 MW on the lower level, heat captured in water at the chip
Indoor baths, rotenburo, private tubs
Two warm pools in the sanctuary
Space heating, hot-water pre-heat
Surplus heat for the hotel and university
Cooled water flows back to the servers
Indicative temperatures for a cascade design — final values subject to the engineering study. A 1 MW hall yields about 0.7 MW of usable heat — about 90 % of the winter peak of the larger house; buffer tanks on the lower level smooth the evening peak and a small heat pump covers the rest.
Looking down on the servers through the glass band at street levelDirect-to-chip liquid cooling turns the data centre into the house’s heating plant — available 24 hours a day, 365 days a year, independent of the weather. The server hall lies half sunk below the car park; every guest looks down on it through the glass band and the glass floor of the reception, so they see where the warmth comes from. At dusk the band glows like a lantern. Dry coolers for the summer peaks sit on the roof behind timber louvres. The hall can be fitted out and run by a data-centre partner who pays rent and delivers the heat, or by YUKI itself.
When the baths need less, the surplus goes to the hotel and the university next door, and dry coolers handle the peaks. Turning 55 °C water back into electricity (ORC) would recover only about 3 % — storing summer heat in the ground for the winter is the more promising route, and part of the engineering study.
Under the EU Energy Efficiency Directive (EU 2023/1791), data centres above 1 MW must make use of their waste heat unless they can show that this is not technically or economically feasible. YUKI is designed around that requirement from day one.
Indoor baths, a sauna world with a panorama sauna facing Vienna, a forest sauna on stilts and an open-air deck at around 42 °C in cedar and stone, looking into the trees and over the sanctuary. Day visits, memberships and private baths.
Japanese snow monkeys bathing in warm springs — visible from the rotenburo and from a public boardwalk for families and schools.
YUKI’s troop would come from Affenberg Landskron near Villach in Carinthia — Austria’s own monkey mountain, where a colony of Japanese macaques has lived in a near-natural forest enclosure since 1996.
A domestic source with three decades of experience in keeping the species — and a short move within Austria instead of animals from overseas.
One Japanese restaurant with an omakase counter for around 50 guests, and a whisky bar for about 20 — on level 3 of the onsen house, open to bathers and to everyone else.
From the top floor and the roof terrace: the Danube, the skyline and the lights of Vienna.
Onsen and sauna day visits, memberships, private baths and treatments.
Sanctuary tickets, school programmes, events and the shop.
Japanese restaurant with omakase counter and a whisky bar with a view.
Rent from a data-centre partner for the server hall on the lower level — or colocation revenue if YUKI fits it out — plus heat sold to the neighbours.
The compute pays for the heat.
The heat creates the experience.
And the experience brings the guests — every single day of the year.
Heating for baths, springs and the house comes from compute operations instead of gas.
With a partner, the base case is about €35 m — above the original €20–30 m target, with clear levers to bring it back; building the data centre ourselves adds compute revenue but roughly €13 m of capital.
Warm water and indoor-outdoor experiences work in every season — winter is the highlight, not the low season.
Snow monkeys bathing in warm springs, a timber onsen and Japanese cuisine, minutes from a European capital.
This is the same model as our Excel workbook. Change any assumption and the investment, the operating result and the payback update instantly. All figures are indicative, net of VAT, at today’s prices.
Build costs include planning, permits, a contingency and financing during construction; land is leased, not bought. Operating costs include staff, food & beverage, energy, maintenance, insurance, the ground lease and animal care. Concept-stage estimates — not an offer and not investment advice.
