Like something from a tropical postcard, Tamul is easily the Huasteca Potosina’s most spectacular waterfall. Milky-blue water plunges 105m into the pristine Río Santa Maria (which becomes the Tampaón). Its setting, in a canyon surrounded on all sides with thick forest, is simply breathtaking. Best of all, you'll often have the place to yourself, due to its remote location.
To see the falls, you can drive almost all the way to the top entrance (you'll need to pay M$20 to drive along this private dirt track). When you reach the river, you'll need to park, wade across to the campsite on the other side (if the river is flowing heavily, it's best to be helped across by one of the campsite employees), from where it's an easy 1km hike to the waterfall itself. You'll emerge at its top, where there is a lovely pool to swim in, but be sure to take the rickety wooden ladder down to the bottom of the valley and see the falls from there, where they are truly a spectacular sight with thousands of butterflies flapping about in the spray.
Another way to reach the waterfall is to paddle upriver in a wooden lancha (boat), a return journey of around 3½ hours (M$800 to M$1000 per boat depending on your haggling skills). You can arrange trips on arrival into Tanchachín or La Morena.
If you don’t have your own transportation, MS Xpediciones arranges excellent day trips from Ciudad Valles, complete with lunch at the home of a hospitable local (M$950 per person including transportation; minimum two people). It also arranges rafting trips in the area.