After a steady descent to 65 feet (20 m), you level off on a large pinnacle and begin to take in the sights. At first, you maintain a constant depth, using the wall as a reference point to your left. Your buddy is right beside you, your NDLs are good and you have plenty of air. Suddenly a strong current, with a force like you’ve never felt, is pushing you downward. You instinctively kick upward as hard as you can against the downcurrent and add air to your BCD. But it’s not working — you’re still sinking. You glance at your dive computer and in a matter of seconds, you’ve dropped to 114 feet (35 m). You’re focusing all your energy on trying not to panic as you equalize your ears. You’ve now fully inflated your BCD, and while doing so has slowed you down, you’re still descending.
Swim Away from the Wall
When you lock eyes with your buddy, you realize that she is beckoning you to follow her away from the wall. As a last-ditch effort you start to kick horizontally, with your body now at a 45-degree angle. Within moments the current subsides and your computer is urgently beeping that you’re ascending too fast. You manage to vent air from your BCD and arrest your ascent. After what feels like a lifetime of getting your breath back, you and your buddy manage a controlled ascent to 15 feet (5 m), where you do a safety stop.
Even though you surfaced far from the pinnacle and your entry point, you brought a DSMB on the dive and deployed it during your safety stop. After your boat spots it and picks you up, you spend the journey back to shore reflecting on a pretty unnerving experience that probably only lasted around a minute, but felt much longer.
What causes a downcurrent?
These scary phenomena commonly occur at underwater pinnacles with steep drop-offs into very deep water, such as walls. Downcurrents are also more likely in areas known for having strong currents in general. There are various theories as to how they form, including one that postulates that when opposing horizontal currents meet, the water cannot be compressed; it has to move up or down, and usually follows the local topography, such as down a wall. Convection currents, whereby cool, dense water sinks into very deep water may also be to blame for downcurrents.
How should you have reacted?
In our example above, you did almost everything you should have done. Being caught in a downcurrent can be unnerving for any diver. The first — and crucial — step is to realize what’s happening and react quickly. If you’re close to the wall you may be able to shelter underneath a ledge. Be aware, however, that you may not be able to simply wait it out. The force of the water may also make it difficult to keep your mask on your face and your regulator in your mouth. Instead, in the short time you have to react, try to swim away from the wall. The single most important thing to do is stay calm. Panic is a sure way to turn a dangerous situation into a deadly one.
Downcurrents are like rip currents in the sense that although they can be ferociously strong, they are not normally very wide. Therefore, it shouldn’t take you long to swim out of it, as long as you swim perpendicular to the flow, i.e. horizontally. Until you do, you will continue to sink. Don’t attempt to swim against the current by swimming upward; this will simply exhaust you and cause more anxiety while you continue to sink. Although adding air to your BCD may provide a larger surface area for the downcurrent to drag you down, the additional buoyancy will help to slow your descent. Just be ready to dump all that extra air when you do come out of it. Dump your weights only as a last resort if the downcurrent has pushed you very deep since doing so could lead to a rapid ascent.
Finally, in the example above, the boat spotted you because you carried and deployed an ocean-going DSMB. This allowed them to quickly come to your assistance.
Is there any way to anticipate a downcurrent?
There may be some tell-tale signs that you’re heading into a downcurrent. Exhalation bubbles traveling downward are an obvious sign. Fans and soft corals seemingly being blown downwards are also an indication, as are small fish swimming erratically up and down.
Finally, always listen carefully to any site briefings. Guides can prepare you for any likely downcurrents and tell you what to do. If you are diving with a divemaster or instructor, always follow their instructions during the dive. They visit the area frequently and are far more familiar with the conditions than you.