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You've heard the grim timelines: if warming continues, the Great Barrier Reef will be bleached by 2030; glaciers in the Swiss Alps, on Mt. Kilimanjaro, and in Glacier National Park will disappear in under 40 years; and Arctic ice melt will leave the North Pole bare and polar bears extinct. The immediacy of these timelines prompts flocks of curious eco-tourists to travel to environmentally fragile areas. Tourism is both bane and boon: it can add strain to already distressed areas, but it can also provide income, which in turn can help preserve these wonders. In time for Earth Day, we spotlight 10 areas under threat—some lesser known than others—that can still be visited responsibly. Should you decide to plan a journey, we've recommended our favorite tour operator for each destination. In some cases the price tag may be higher than your average vacation, but consider it an investment in Mother Earth.Natural Wonders to See Before They Disappear Going once, going twice…some of the world's most enchanting places might not make it through this century. Here are 10 places you may not have realized are threatened—and how to experience them responsibly.