The karst country inland from Dubrovnik is a landscape of grey limestone hills, stony fields and long, flat-bottomed valleys called poljes, which flood in winter and dry out in summer. Popovo Polje, on the Bosnian side of the border, is one of the largest, and at its edge, near the village of Zavala, lies Vjetrenica, the most famous cave in Bosnia and Herzegovina. Its name means wind cave, from the strong current of air that blows out of its entrance, cool in summer and noticeable from a distance.
Vjetrenica is a long cave system, with several kilometres of passages explored, and it is renowned among biologists as one of the richest caves in the world for cave-dwelling animals, with many species found nowhere else. Among them is the olm, a blind, pale salamander that lives in underground waters in the Dinaric karst. The cave is on the tentative list for UNESCO World Heritage. A section of the cave is open for guided tours, with lighting and walkways, and the temperature inside is cool all year round, so bring a layer.
Zavala is a small village with a monastery, and the area is sparsely populated, with stone houses, dry-stone walls and the occasional vineyard. Trebinje, the main town in the area, is a pleasant place on the Trebišnjica river, with a stone bridge, plane trees and the Tvrdoš monastery, which produces wine. Popovo Polje is known for its wines, both white and red.
The riding in this area is wonderful and quiet. The roads from Dubrovnik climb steeply into the hills and cross the border into Bosnia, then wind across the karst plateau, with long views and almost no traffic. Surfaces vary, and some roads are narrow. The border crossings can involve queues in summer, and you need a green card for insurance. Be aware that some areas in Bosnia still have unexploded mines from the war of the 1990s, so stay on roads and marked paths.
Fuel is limited in the hills; Ljubinje, Trebinje and Dubrovnik, about twenty-five kilometres away, are the nearest options. Spring and autumn are the best seasons.