By the Huangyanghe Reservoir (黄羊河水库, Huángyánghé Shuǐkù), it's hard to appreciate how massive the 15m-high Shakyamuni Buddha statue at Tiantishan Grottoes is until you reach his truck-sized feet to peer up at his outstretched hand emerging from the cliff face. These 1600-year-old carvings stand majestically in the open air, but the real star is the Buddha, his enormous feet protected from the reservoir's flood by a giant, half-moon dam around which you can walk to see him from varying vantages.
There are 17 caves here containing ancient murals (tigers, black dragons), along with some scroll paintings. Only one is open to the public, however, as many suffered devastation after a large earthquake in 1927. In 1959 many of the relics from lower caves were moved to Gansu Provincial Museum to make way for the construction of the reservoir. Two sets of stairs also lead to the dam floor, allowing worshippers to descend and light incense.
To reach the grottoes, take a bus (¥10; first/last bus 7am/5.30pm) to Haxi (哈溪, Hāxī) and tell the driver you are heading to the grottoes; he will tell you where to get off, then you will need to walk around 2km to the grottoes. To return to Wuwei, walk back to the drop-off and wait for a bus back (the last bus returns at 5.30pm). Alternatively, hire a driver or take a taxi from Wuwei (¥200 per half-day).