On my most recent trip to Tobago, I racked up two underwater adventures that had long been on my to-do list. The first was a visit to St. Giles, the uninhabited island that lies just north of Tobago, and a dive under the nearby pinnacle known as London Bridge. Because St. Giles sits in open water, exposed to the full force of trade-wind swells rolling in from the Atlantic, dive operators have to pick their days to make the five-mile open-water crossing from Speyside. The day we went, swells were moderate - 5 to 8 feet - and our slender, pirogue-style dive boat handled them with ease. Riding a small open boat into an active ocean is not for everyone, but our group managed to avoid seasickness and agreed that the trip would make a good adventure story in our logbooks. London Bridge is actually a pair of rock pinnacles joined by an arch that extends some 30 feet above the water's surface. A constant surge washes through the narrow gap below the bridge, creating anticipation and maybe just a little anxiety for those of us contemplating a swim through the foamy gap. As with most dives around Tobago, the strategy for success and survival meant following the divemaster's lead. The local divers understand the intricacies of the currents, eddies and rips that swirl and sweep the island's shore, and they know which routes will keep you out of trouble. Underwater, London Bridge was not as dramatic as expected. The slopes of this volcanic pinnacle are similar to others in the area, such as the Sisters, but on this day, there was no big marine life to be found. The wow factor increased when we turned and entered the swim-through, which is a canyonlike slit framed by two massive rock slopes. At 20 feet deep, I could feel the surge impact the rocks above, then rebound and regroup for another attack. Once we went through the bridge, conditions calmed considerably, and we spent a mellow half-hour drifting in a protected lee cove filled with schools of durgeon and jacks. During the surface interval, the crew moved us in close to the wooded cliffs of St. Giles, which is home to thousands of nesting sea birds - definitely a high point of the trip. Adventure No. 2 took place on the ride home. Just past Starwood Bay, we came across a trio of juvenile manta rays feeding in a rip where inshore and offshore waters met. After years of hearing about Tobago's fabled mantas, this was my first actual encounter. We jumped in with mask and snorkel but were mindful of the mantas' personal space. Years ago, divers would latch on to the adult mantas that were once common in the Speyside region; hitching a free ride was known as catching a "Tobago taxi." For whatever reason, manta sightings declined in recent years. Some say it was illegal longline fishing offshore, but I suspect that irresponsible diving practices may have had a role as well. Whatever the cause of the historic decline, the mantas are starting to show up once again, and I'm looking forward to more encounters in the years to come - but no rides.