In the delta country east of Hanoi the land is mostly flat, a patchwork of rice paddies and ponds, so when limestone hills rise abruptly out of it they feel like a different world. Kinh Môn, in Hải Dương province, is one of those places, a small range of karst hills pitted with caves, and the Kinh Chủ cave, which I have placed under the name on the map, is one of them. Caves in northern Vietnam are often turned into shrines, with Buddha statues and incense in the dim light, and visitors come both for the religion and for the cool air inside.
Do not expect a quiet wilderness. This is a working landscape, and cement plants have been cut into these limestone hills for generations, so dust, trucks and industrial sites sit alongside temples and rice fields. Even so, the contrast is memorable: a monk's chant in a cave, the green glare of paddies outside, and the river Kinh Thầy running muddy and busy with barges. The pagoda in the same ride, Thanh Mai, is associated with one of Vietnam's Buddhist traditions, and you will often find locals leaving offerings there on feast days.
For riding, northern Vietnam rewards the patient. Highway 5 between Hanoi and Haiphong is busy and loud, full of motorbikes, lorries and buses, and the secondary roads are often better for the experience, with village markets, dikes and bridges over canals. Expect surprises: a water buffalo on the road, a sudden stall of fruit, a crowd of schoolchildren on bicycles. Surfaces are usually paved but can be uneven, and rain turns the shoulders to mud. Spring is mild and damp, summer hot and wet with typhoons possible, and October to December is generally the most comfortable.
Hải Dương, about twenty-one kilometres away, is large enough for petrol stations, hotels and restaurants, and it is famous for a mung-bean cake called bánh đậu xanh that you will see in boxes in shops everywhere. Fill up before you head out to the hills, since village pumps can be few. Take a head torch if you plan to go into the cave, wear shoes with grip, and leave a small offering or donation if the keepers ask.