Baikal (байкл) is a Russian carbonated soft drink that was first produced in 1969 as a sort of Soviet knock-off of Pepsi Cola. In the early 1970's, Pepsi officially entered the USSR market, and Baikal was reformulated into a more herbal soda which, while still retaining some cola-like properties, was less Westernized and more in line with traditional Russian tastes. Forty years later, Baikal is a top-selling flavor in Russia and a popular drink in Russian markets here in the US.
The taste is hard to describe, and most brands of Baikal aren't any more helpful in terms of ingredients than American sodas, most of them listing only "natural herbal flavoring BAIKAL" on the label. I've heard people describe it as "kind of like Dr. Pepper," though to me it has a lot more in common with Moxie than it does Dr. Pepper. The first time I tasted it, I noticed a whiff of eucalyptus - not overpowering, or heavy in an "air freshener" or "cough drop" way, but used rather as a flavor accent that helped bring out other pleasant, earthy tastes. As it turns out, "natural herbal flavoring BAIKAL" contains essential oils from bay laurel, lemon, eucalyptus, St. John's wort (hypericum), Russian licorice, and Siberian ginseng.
I find it very refreshing; Baikal isn't usually as sweet as American soda pop, and the herbal extracts offer a novel flavor profile. It kind of reminds me of old, traditional brewed root beer, back when root beer was made of roots and birch shoots rather than artificial flavors and corn syrup. While it's not as easy to find as tarkhun, if you find it I recommend picking up a bottle to try.