Diving with whales is one of the most extraordinary experiences the planet has to offer; take this activity to a little-known French Polynesian isle, and it becomes an absolutely incomparable adventure. Rurutu, the gem of the Austral archipelago, is dotted with limestone caves and is steeped in culture and honest smiles. While the weather in Rurutu is often warmer from November to March, the best time to visit is between July and October, when migrating humpbacks come close to shore to reproduce before heading back to icy Antarctic waters.

Geologically, Rurutu is a makatea: an uplift of coral reef and volcanic cone that forms a lush, volcanic interior with a limestone perimeter gouged with caves. Because of this, the topography is startlingly different from your common volcanic island. Sheer cliffs of pock-marked coral rise up around the coast, sometimes housing giant caverns filled with ferns and stalactites. The island’s interior is a lush, tropical paradise. The island is fringed by a short continuous reef, which drops directly into startlingly clear, deep ocean.

Rurutu is becoming renowned, not for the quantity of whales that pass its shores, but for the quality of the sightings. Because the water is uncommonly transparent, it’s easy to dive with them from a safe distance (so as not to scare them away or put yourself in any danger) while having phenomenal visibility. The lack of a barrier reef means that the whales come in close to shore, making them easier to locate. The dive companies do not promise that you will see whales and there is no refund if you don’t find any. The best chances of seeing whales are in August and September, and it’s best to book hotels and flights well in advance since facilities are limited.

When not kicking around looking for whales, there are plenty of other activities to keep you busy. Easy access limestone caves include the Mitterrand cave, where Rurutuans performed a welcome dance for the ex-French president. Guided spelunking and walks are available through knowledgeable Reti Mii (011-689-940-538; from $20), while not-to-miss horseback-riding tours through the mountainous interior ($50 for a half-day) can be arranged through Viriamu Teuruarii at Pension Teautamatea.

Be sure to pack a few sweaters and warm socks — Rurutu is only slightly north of the Tropic of Capricorn, and temperatures can get surprisingly chilly for the tropics. Local food is based on taro (a starchy root tuber), fish and pork, and the food served at all the lodging options is delicious and copious.

Diving: Raie Manta Club (011-689-968-560; www.raiemantaclub.free.fr; half-day whale tour $100) has been offering very professional whale tours for more than nine years, while newer Rurutu Baleines Excursions (011-689-940-791; half-day whale tour $90) is trying to make a name for itself. The Raie Manta Club also offers non-whale-oriented exploration dives around the coast.

Where to stay: The newly remodeled, upscale Rurutu Lodge (011-689-940-215; www.rurutulodge.com; bungalows $150) is run by the Raie Manta Club dive center. Beautifully designed with authentic Polynesian detail, this is by far Rurutu’s most comfortable choice, although you’ll find less contact with the locals here than anywhere else on the island. For a more authentic experience, stay with the Rurutu hiking specialist and his charming family at Le Manotel (011-689-930-226; www.lemanotel.com; bungalows from $60; half-board $32 per person). The enchanting private bungalows are squeaky clean, nestled in a blooming tropical garden and are across the road from a long stretch of beach. You can really live with the locals by staying at Pension Teautamatea (011-689- 930-293; www.teautamatea.com/en; double including half-board $120 per night), a lively family home amidst a coconut plantation, roaming horses and an archaeological site (Marae Tararoa); a deserted white beach is just across the street.

(Celeste Brash has lived in French Polynesia for more than 11 years and is co-author of Lonely Planet’s guidebooks to French Polynesia and Tahiti, as well as various other titles.)