10 great places on Earth you don't want to miss

— -- In anticipation of Earth Day on Wednesday, now is a good time to focus on some of our planet's unique — and endangered — travel destinations. Holly Hughes, author of the new Frommer's 500 Places to See Before They Disappear, shares some with Tim Smight for USA TODAY.

Mount KilimanjaroTanzania Rising majestically to a height of 19,330 feet, Mount Kilimanjaro is Africa's tallest peak. But its famed snow cap is shrinking. "More than a third of it has melted since 1990," Hughes says. "A combination of reduced snowfall, evaporation and internal heat from the dormant volcano are to blame." Guided excursions are available for those who would like to climb this icon before the snows vanish. tanzaniaparks.com/kili.html

Angkor Wat Siem Reap, Cambodia The capital of the Khmer kingdom from 802 to 1295, Angkor Wat is Cambodia's chief tourist attraction — temples and shrines that cover 38 square miles. "More than 1 million tourists visit this ancient city every year, putting tremendous stress on the sandstone temples, stairs and walkways," Hughes says. "It's one of the world's most endangered classical sites, and the Cambodian government may have to impose stricter quotas or even close certain buildings in the very near future." tourismcambodia.com/attractions/angkor

Adirondack State Park Upper New York state Established in 1892, this 6 million-acre park encompasses more than 3,000 lakes and ponds connected by 1,500 miles of waterways. "The effects of acid rain, encroaching development and harmful invasive species are taking a toll here," Hughes says. "But much of the park's heart has been kept inaccessible to vehicles, preserving a slice of wilderness. The best way to appreciate it is to canoe through its quiet rivers and forested lakes. You'll see white-tailed deer, beaver, and, if you're lucky, you may spy a red fox or even a moose." 518-846-8016; adirondacks.org

The EvergladesSouth Florida A waterworld of mangrove swamps and grassy wetlands, the Florida Everglades is home to rare plant and animal species including manatees, hawksbill turtles and American crocodiles. "An estimated 50% of this unique ecosystem has already been lost to development," Hughes says. "Dwindling water levels and pollution are compromising what remains." nps.gov/ever

Valley of the Kings Luxor, Egypt More than 60 subterranean crypts of ancient Egyptian pharaohs have been unearthed at this legendary archaeological site. But the site's survival is precarious: sewage runoff and irrigation have affected the underground reservoir beneath the crypts, eroding their stone foundations. "The tombs are still open on a rotating basis," Hughes says. "If you're lucky, you may be able to see the burial chambers of King Ramses VI or King Tutankhamen." egypt.travel

Michoacán Monarch Biosphere ReserveMexico Each year, tens of thousands of monarch butterflies complete a remarkable 2,000-mile migration between Canada and their winter nesting grounds high in the mountains of northeast Mexico. "Stepping into a grove of monarch-laden fir trees is like stepping into a kaleidoscope," Hughes says. But it's an experience that may soon be a memory: Nearly half the reserve's forest canopy has degraded because of relentless logging. "The reserve is reachable by day trip from the Colonial-era city of Morelia — allow 10 to 12 hours for a guided excursion." michoacanmonarchs.org

The Dead Sea Israel Surrounded by desert ridges interspersed with lush oases, the Dead Sea presents a visually stunning tableau. But with the upstream Jordan River increasingly diverted for irrigation, the water level of this huge inland lake has been dropping at an alarming rate, Hughes says. "Scientists estimate that within three decades, the Dead Sea could be completely dry." deadsea.co.il

The Nazca Lines Peru One of the most intriguing and mystifying ancient sites — believed to have been created by the Nazca, a culture that flourished around 300 B.C. — is on a high, arid plateau about 300 miles south of Lima. A hummingbird, spider, condor and a monkey are among the more than 30 elaborate stylized figures etched into the plain. Hundreds of feet in diameter, the drawings can only be seen in their true dimensions from the sky. "The region's arid climate has helped preserve the Nazca Lines for centuries," Hughes says. "But the site faces serious threats, most critically from erosion caused by deforestation and global warming." visitperu.com

Piazza San Marco Venice "Sitting in the very heart of Venice, Piazza San Marco is arguably the loveliest public space in the world — a huge open square fronted by the magnificent St. Mark's Basilica," Hughes says. "But since the 1980s, the piazza has been underwater more than 40 times per year." The time to see it may be growing short. "Scientists estimate that this iconic Italian Renaissance city is sinking at the rate of 2½ inches a year, and rising sea levels in the Adriatic make flooding a constant concern." www.comune.venezia.it

Overland Track Tasmania Off the coast of southern Australia, Tasmania encompasses one of the world's more unusual ecosystems. Running through the island's center like a spine is the Overland Track, a 40-mile trail that traverses high alpine plateaus, marshy plains, eucalyptus groves and one of the planet's last temperate rain forests. "Many unique species can be viewed along this famous trail, including the island's 'mascot' — the sharp-snouted little scavenger known as the Tasmanian devil," Hughes says. "But logging, mining and industrial development are seriously threatening the wildlife habitat here." www.overlandtrack.com.au