The park exists because of a single administrative act: Decision 33, dated 27 January 1992, issued by the Chairman of the Council of Ministers of Vietnam. It drew a line around 14,735 hectares of the Thanh Hóa uplands between roughly 19°31′ and 19°43′ north, spanning the hill country of Như Thanh and Như Xuân, some 46 kilometres southwest of Thanh Hóa city. Of that total, 8,544 hectares — more than half — is primary forest that was never cleared, which in northern Vietnam is a genuinely unusual survival after a century of war, logging and land hunger. The park is managed under IUCN category II, the same protection standard applied to national parks worldwide. Inside the boundary the terrain rises into hills and mountains, cut by streams and rivers that all eventually drain the same way.

They drain into Sông Mực, and Sông Mực is the reason most visitors come. The lake covers around 3,000 hectares behind a dam, and when the water rose it did what reservoirs do in hill country: it flooded the valley floors and left the ridge tops standing. Twenty-one islands are the result — not islands in any geological sense but hilltops that stopped being hilltops. The shape of the water is therefore the shape of the old valleys, branching and doubling back, and you read the drowned landscape by following the shoreline. Motorboats and kayaks both work. Caves open in the park's northern and southern sections. Of the two forest trails, one starts near the guesthouse by the dam and the other runs across one of the islands, which also holds a pair of holiday houses available to rent — accommodation on a hill that used to have a valley beneath it.

Surveys here have produced a long ledger: 1,389 species of plants and 1,004 of animals, broken down into 66 mammals, 201 birds, 54 reptiles, 31 amphibians, 68 fish and 499 insects, with 462 varieties of flora across 125 orders. The endangered ironwood Erythrophleum fordii, cut nearly out of existence across its range for timber that resists rot and insects, still stands here. But the record keeps an unusual piece of honesty in it. Some of the species on that list, the surveys note, are probably already gone from the park — counted once, never seen since. A biodiversity inventory is normally presented as an asset register. This one reads more like a memorial with an uncertain number of names on it, and the uncertainty is the point: nobody can yet say which entries are still true.

Thái, Thổ and Mường communities live within the national park. That sentence sits quietly in the record and carries a great deal. These are upland peoples who were farming and fishing these valleys long before a decree in Hanoi reclassified the ground beneath them, and the 1992 line was drawn around them rather than through them. It leaves the park with the situation that most protected areas in mainland Southeast Asia share: a landscape that is simultaneously a conservation category, a watershed, a tourist destination, and somebody's home village with a school and a graveyard in it. The forest that the decision of 1992 set out to protect is not wilderness in the sense the word usually implies. It has been lived in, and the living is part of what survived.

In 2011 the park recorded 6,570 visitors across the whole year — an average of eighteen a day, which at Vietnam's better-known parks would barely register as a slow morning. The infrastructure matches: a guesthouse near the dam offering basic rooms, two rental houses on an island, two trails, a boat. Bến En has never been a headline destination, and the obscurity has been, on balance, a form of protection. It also means the place still delivers something the crowded parks cannot — a lake with twenty-one islands on it and, most days, almost nobody else looking at them.