He graduated in Physics at the University of Rome in 1902. Assistant at the "Regio Ufficio Centrale di Meteorologia e di Geodinamica" (Italian Agency of Meteorology and Geodynamics) from 1905 to 1927. Teacher of Geophysics from 1915 to 1925 at the University of Rome, then professor of Experimental physics at the University of Bari from 1928.
It had been observed that an electroscope in a vessel at earth potential gradually lost its charge, even if very carefully insulated, due to the ionization of the air. Pacini observed simultaneous variations of the rate of ionization on mountains, over a lake, over the sea, and underwater. In an experiment performed in June 1911, Pacini concluded that ionization underwater was significantly lower than on the sea surface. He could then demonstrate that a certain part of the ionization itself must be due to sources other than the radioactivity of the Earth, thus contributing to the discovery of cosmic rays.
- Alessandro De Angelis (2012). L'enigma dei raggi cosmici. Springer.
- D. Pacini (1912). La radiazione penetrante alla superficie ed in seno alle acque. Il Nuovo Cimento Serie VI, Tomo 3: 93-100. [Translation http://arxiv.org/abs/1002.1810]
- P. Carlson and A. De Angelis (2010). Nationalism and internationalism in science: the case of the discovery of cosmic rays. European Physical Journal H 35, 309-329 (2010). 
- University of Pavia, Biography of Domenico Pacini
- A. De Angelis et al., "Domenico Pacini, un pioniere dimenticato dello studio dei raggi cosmici", Nuovo Saggiatore 24 (2008) 70.