It's just after 5pm on a tropical afternoon as three other divers and I splash noisily into the sea and descend slowly to the reef below us. We find a clear space on the seabed and settle down facing an innocuous-looking pile of coral rubble. As the sun sinks, we wait in the gathering underwater gloom. Suddenly, there's movement among the coral, and a tiny red and green fish pokes its head out of a hole. It's followed by another, slightly bigger, and then two or three more of different sizes. As we watch, the bigger fish perform a courtship dance, teasing out the smaller females.
At almost exactly 5.30pm, these tiny fish, no larger than my little finger, pair off and dart up from the reef, mating briefly a few inches from our masks before scurrying away to the safety of their coral homes.
What I have just witnessed is the courtship and mating of mandarin fish. It took only a few seconds but it's something I've never seen before in 25 years of diving. This is what diving in North Sulawesi is all about: the opportunity for rare encounters, special meetings with some of the most wonderful and bizarre marine life on the planet.
The Bunaken National Marine Park, off-shore from the coastal town of Manado in North Sulawesi, is one of the focal points for diving in this region. Covering more than 185,000 acres around a cluster of small islands, the park is home to some of Indonesia's finest coral reefs. It lies at the epicentre of the riches of Indo-Pacific marine biodiversity.
Bunaken is one of the great success stories for marine parks in Asia - or, indeed, globally. "The amount of fish has increased dramatically in the past five years," says Christiane Muller of Froggies Divers, based on Bunaken Island. "And we're also getting new fish that we didn't see before." Froggies is at the heart of the marine park, with easy access to some of the best and most spectacular dive sites.
Siladen Island also lies within the marine park. The resort has the best beach within easy reach of Bunaken's reefs, but you can't really swim because it's too shallow (although there is a great swimming pool). It's a small, intimate place with an attractive, open-sided bar/restaurant area and spa centre. The villas are very romantic, with four-poster beds and open-air bathrooms. The dive centre is well run, with most Bunaken dive sites less than 10 minutes away; the house reef has spectacular shallow-water corals.
The proof of Bunaken's conservation success is in the diving: this is a fabulous, once-in-a-lifetime experience, well worth the long journey to get here. Sheer reef walls, resplendent with coral, plunge down into the depths. Trevallies, bannerfish, angelfish, snapper, butterflyfish and many more species flash their colours against a backdrop of sponges, gorgonians, anemones and hard and soft corals. Tuna, turtles, rays, sharks and jacks can be spotted out in the blue. Pilot whales, dolphins, and whale sharks are also sometimes seen. Even orcas sometimes pass through.
There is also first-class snorkelling: the shallow reef tops are glorious, every square inch packed with life from colourful corals to encrusting sponges, crinoids, sea fans, and other plants. Swarms of anthias and basslets dart here and there in the shallows, while the bigger schooling fish flow like rivers of liquid colour over the reef crest and down into the depths.
All this is even more amazing given that you can still see bombed-out areas where dynamite fishing had destroyed entire sections of the reef: the vibrant, living reefs which now thrive in most of the park are testament to the huge success of the Bunaken Marine Park. More than £1,000 a year is channelled back into each village in the park from divers' fees. All divers and snorkellers in the park pay $17 (£8.70) for an entrance pass, which funds ranger patrols, village improvement programmes, waste disposal, and other conservation projects.
Outside of the marine park, there's excellent diving and snorkelling around the northern tip of Sulawesi, accessible from resorts such as Gangga Island. This Italian-managed resort, three miles from the north coast of the mainland, has a gorgeous beach and a terrific pool. The 30 chalets, with verandahs overlooking the beach, are set in luxuriant tropical gardens. Standards of food and service are high: this is the top choice for island luxury and has an excellent spa. There's great diving and snorkelling within easy reach of the island, and they run regular trips to Bunaken. You can walk through neighbouring villages and get a feel for island life, which you can't do elsewhere.
The other main diving area is in the Lembeh Straits, on the eastern side of the mainland, which is famous for its "muck diving"; this involves exploring the seabed looking for some of the extraordinary marine critters that live there. With ghost pipefish, hairy frogfish, pygmy seahorses and orang-utan crabs, this is heaven for macro-photographers and ichthyologists.
The Eco-Divers operation at Kungkungan Bay in Lembeh Straits is efficient and friendly, however the resort itself is tired-looking; boat traffic in the straits is noisy, and the beach is black sand. I'd recommend it only if you're passionate about muck diving.
Despite Bunaken's worldwide reputation, diving here is still very low key and dive sites are uncrowded: there are only 2,000 or so divers per month, spread across more than 100 dive sites. Bunaken does have some strong and unpredictable currents, which means it's not suitable for novices.
North Sulawesi is a fascinating region, and it's worth taking the time to explore inland as well as underwater. It's worth making a day trip to the Minahasa Highlands (see 24 Hours, below), as well as visiting the Tangkoko Natural Reserve on the north-west coast, about 90 minutes from Lembeh or Manado. This is a little gem of a nature reserve alongside the beach. The rainforest is home to crested black macaques that live partly on the ground and are quite used to humans since there's a research station based here. As a result you can stand among them as they carry on with their daily round of foraging, feeding and socialising.
Further into this magnificent forest you may get a rare opportunity to see the world's smallest primate, the tarsier spectrum. These cute little creatures, no more than six inches tall, live in pairs and eat only insects: your guide will collect insects so that the tarsiers come out to feed.
A predominantly Christian area, North Sulawesi is dominated by large, elaborate churches that have been built in even the smallest of villages. And everywhere you go, you'll be met by warm smiles and given a big welcome.
THE COMPACT GUIDE
HOW TO GET THERE
Nick Hanna travelled as a guest of Regaldive (0870-2201 777; regaldive.co.uk), which offers seven nights' b&b at Tasik Ria from £989. Diving costs from £245 for five days (including three guided dives per day, lunch, tanks and weights). Seven nights' full board at Kungkungan Bay Resort (kungkungan.com) costs from £1,129, with diving costs starting from £200 for 10 dives. Seven nights' full board at Siladen (siladen.com) costs from £1,280, where 11 dives costs £198 including tanks and weights. All the above prices include return flights, transfers, and a one-night room-only stop-over in Singapore.
Gangga Island (ganggaisland.com) is featured by Snooba Travel (0870-162 0767; snooba.com), which offers 10 days from £1,450 including return flights, transfers, nine nights' full board, 16 dives, and a one-night stop-over on the return journey. Non-divers start at £1,170.
Clients at Froggies (divefroggies.com) are usually independent travellers. Other dive operators for North Sulawesi include Dive Worldwide (0845-130 6980; diveworldwide.com) and Explorers (0845-644 7090; explorers.co.uk).
The North Sulawesi Watersports Association is very active in supporting the marine park and working on conservation projects. For details visit DiveNorthSulawesi.com.