"It's nature's magical design," says a tourist guide, waxing poetic as he comments on the impressive red cliffs plunging into a turquoise sea at the Scandola nature reserve on France's Corsica island.
"Amazing!" exclaims Irena Snydrova, a Czech tourist visiting the UNESCO World Heritage site with her family, along with groups from Italy, Spain and France.
Their boat sidles up to the Steps of Paradise, rocks shaped into a stairway some 15 metres (50 feet) long, then glides on to Bad Luck Pass, a former pirates' redoubt.
The ages have sculpted the volcanic cliffs into myriad shapes that beguile the visitor, who might imagine a kissing couple here, a horse's head there, Napoleon's two-cornered hat further on...
The park, created in 1975, is an ecological dream, being a nature reserve and a protected marine zone that is listed by France's coastal protection agency and Natura 2000, in addition to its recognition by UNESCO.
It is a prime destination for the some three million people who visit Corsica each year, 75 percent of them in the summer.
The paradox is that growing numbers of tourists are drawn to Scandola's pristine waters and stunning geological vistas, endangering its fragile ecosystem.
The park, reached only by boat some 40 minutes from the tiny port of Porto, stretches over 10 square kilometres (nearly four square miles) of sea, and a somewhat smaller area of land.
"The reserve is a jewel for Corsica and the Mediterranean, but several red lights are flashing," says marine biologist Charles-Francois Boudouresque, listing flora and fauna at risk, including ospreys, seagrass and fish species such as the brown meagre.
The tourist season coincides with the ospreys' mating season, notes Boudouresque, an emeritus professor at the Mediterranean Institute of Oceanography.
Because of over-tourism, ospreys' "reproductive success is zero or near zero, with either no chicks or just one chick" per year, he says.
Boudouresque, who also heads Scandola's scientific advisory council, says the osprey could become extinct in 50 years.
- 'Golden egg' -
Since last month, at the urging of the scientific council, boats must keep a distance of at least 250 metres from ospreys' nests during the breeding season.
"It's a good start," Boudouresque says.
As for the marine park's fish species, Boudouresque says he thinks the thrumming of the tourist boats is scaring them away.
But a crew member, who gave his name only as Diego, blamed groupers for the declining population of corb. "They eat everything," he told AFP.
Tensions have arisen pitting tour boat operators and fishermen against the reserve's conservationist Jean-Marie Dominici.
Boudouresque says the seagrass "is not in the best shape," blaming the anchors dropped by the many boats -- some of them private vessels without authorised guides.
"It's bizarre for a nature reserve to see all these boats," said Pierre Gilibert, a 65-year-old doctor, who is a regular visitor. "It might be wise to allow access only to professional boats."
Many share the opinion that private boats are not sufficiently monitored or informed of ecological concerns.
"This morning we saw people climbing on the rocks and berthing their boats in narrow passageways, which is not allowed," said Gabriel Pelcot, chief mechanic on a cruise ship of the Corsican company Nave Va.
Nave Va, as well as rival Via Mare, uses hybrid vessels: they are powered by diesel up to the edge of the marine park, then switch to electric for a quieter and less polluting presence.
Pelcot notes that this green option is 30 percent more expensive, but he expects it to catch on.
"We must find a compromise between the need for tourists to enjoy this natural treasure and that of not killing the goose that laid the golden egg," Boudouresque says.
The marine biologist is optimistic that general awareness of the problems is growing.
He envisions ways to marry tourism with preservation. One example, he says, would be to focus cameras on ospreys' nests so that they can be observed without being disturbed.