haunting/dark classical

new to classical, I really like Samuel Barber's Adagio for Strings, Op 11. I am looking for other music with the same dark/haunting sound. Suggestions?
Check out Hovhaness-Symphony No. 2 ("Mysterious Mountain") and Gorecki-Symphony No. 3. Both are dark and Hovhaness in particular reminds me of Vaughan Williams. Enjoy the music.
A little off the beaten path and more recent than those already mentioned; Silvestrov(sp?) from the Ukraine. Dark and brooding but not as "thick" as above mentioned romantics. I believe there are at least 3-4 CDs available.
Olivier Messiaen
"Quartet For The End Of Time"

Its inspiration, alone, meets your interest. The music even more.

You might try this CD (Greif: Sonate de Requiem, etc / Bertrand, Amoyel, Weithaas). I have it and I need to be in a very special mood to listen to it. Shostakovich as well as Prokofiev are also capable of some very dark melodies.

A few of my favorites.

Elgar's Cello Concerto would be similar in mood to Barber's Adagio and is every bit as good IMHO, well almost. You might also like Barbers Violin Concerto.

I would also vote for Sibelius' Sym #4. For something a little different try Death and Transfiguration by Strauss. It is elevating and beautiful.