Herculean, abstract, arcane, arduous, beclouded, blind, brutal, buried, civilized, close, clouded, complex, complicated, concealed, covered, covert, critical, cultivated, cultured, deep, delicate, demanding, difficile, difficult, eclipsed, educated, encyclopedic, erudite, esoteric, exacting, formidable, hairy, hard, hard-earned, hard-fought, heavy, hermetic, hid, hidden, hypothetical, ideal, in a cloud, in a fog, in eclipse, in purdah, in the wings, incommunicado, intricate, jawbreaking, knotted, knotty, laborious, latent, learned, lettered, literate, mean, mysterious, no picnic, not easy, obfuscated, obscure, obscured, occult, operose, pansophic, polyhistoric, polymath, polymathic, profound, recondite, rigorous, rough, rugged, scholarly, scholastic, secluded, secluse, secret, sequestered, set with thorns, severe, spiny, steep, strenuous, studious, thorny, ticklish, toilsome, tough, transcendental, tricky, under an eclipse, under cover, under house arrest, under wraps, underground, unknown, uphill, wicked, wise, wrapped in clouds