Complex sentence: Pre-Scythian-Saka-Sibirian kurgans were surface kurgans and underground wooden or stone tombs constructed on the surface or underground and then covered with a kurgan.
Split into simpler sentences: Pre-Scythian-Saka-Sibirian kurgans were surface kurgans. Underground wooden or stone tombs were constructed on the surface or underground and then covered with a kurgan.