The Altamira cave has reopened to a handful of visitors, giving them a glimpse of 14,000 year-old bison and other examples of spectacular ancient art.
Renowned for vivid paintings of beasts and animal-headed humans, the Altamira cave closed in 2002 because human breath and microscopic fungi introduced by visitors threatened to ruin the paint.
On Thursday it reopened for five lucky visitors, chosen by lot from ticket-holders at a nearby museum that houses replicas of the paintings at Santillana del Mar, in the Spain's far north.
"It is very impressive. You see all sorts of details... they seem to stare at you from out of the darkness," journalist Javier Ors said after visiting.
"There is a life-size deer which looks like a female that is pregnant. That was impressive."
Another visitor, Andrea Vicente, said she was "very moved" by the experience.
"It gives you goose bumps," Vicente said.
The five crept in wearing white masks and overalls and closed the door behind them as they headed underground to see the ancient masterpieces.
Experts arranged the tour as an experiment to assess the impact on the paintings from readmitting the public after 12 years of studies.
The culture ministry said scientists would monitor the temperature, humidity, carbon dioxide and any risk of contamination after the visits.
Unesco listed the paintings as a World Heritage Site in 1985, as "masterpieces of creative genius and as humanity's earliest accomplished art".
By August 192 visitors would enter, and the impact measured, the culture ministry said.