I am seeking the name to this park:  This national park features an
underground waterway that winds 8.2km through a spectacular cave
before emptying into the sea.  I don't know whether it is a US
national park or in another country.

Thank you!

I am confident that the Puerto Princesa Subterranean River National
Park, in the Philippines, is the answer to your question.

"The Puerto Princesa Subterranean River National Park is a World
Heritage Site which features an 8.2 km navigable underground river,
reputed to be the world's longest that winds through a spectacular
cave before emptying in to the South China Sea. Cathedral chambers,
wide hallways, interesting geological formations."

Philippine Convention & Visitors Corporation
http://www.dotpcvc.gov.ph/starstudded/wowpuerto.html

"The most popular tourist attraction in the city [Puerto Princesa] is
the Puerto Princesa Subterranean River National Park despite its
remote location. Visitors usually head to the world-famous
8.2-kilometer underground river at once, for a 30-minute paddle boat
ride inside an enchanting cave filled with bats and interesting
formations. The park covers 22,202 hectares of forests, mountains and
beaches on the western coast facing the South China Sea."

Palawan's Digital Domain: Puerto Princesa
http://mobilemediaph.com/palawandigital/destinations/puertoprincesa.html

"The Puerto-Princesa Subterranean River National Park (PPSRNP) is
located in the Saint Paul Mountain Range. It is 80km north-west of
Puerto Princesa, the capital of Palawan Province. The National Park is
accessible from Puerto Princesa by road and boat.

The PPSRNP features a spectacular limestone karst landscape with its
8.2km long underground river. A distinguished feature of the river is
that it emerges directly into the sea, and the lower portion of the
river is subject to tidal influences."

Shirakami-Sanchi International Conference on World Natural Heritage
http://www.sicwnh.com/English/participant/philippines.html

My Google search strategy:

Google Web Search: "national park" "8.2 km"
://www.google.com/search?hl=en&q=+%22national+park%22+%228.2+km%22

Google Web Search: "Puerto Princesa Subterranean River National Park"
://www.google.com/search?hl=en&q=%22Puerto+Princesa+Subterranean+River+National+Park%22

I hope this helps. If anything is unclear or incomplete, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud

Fabulous answer!  Much more than I expected.  I tried Google, but put
"cave" or "cavern" in my searches, and thus didn't get it.  Well worth
the few dollars to search.  This was the trivia question in a catalog
for a small outdoor equipment outlet company, and getting it right
saves me $10 in shipping.  So it is worth the Google answers price and
a tip, as I am still coming out a few dollars ahead, and I learned
where this was -- it was bugging me!  Your answer makes me want to
travel to see the park!

J.

Thank you!

Thank you very much for the five-star rating and the nice tip!

~pinkfreud