It is crystal clear. When there is no oxygen, but the cells need energy, fermentation takes place. Dr. Otto Warburg got a Nobel Prize for this discovery. But there are hundreds of modern studies that found the same. Here are just titles!

Brizel DM, Scully SP, Harrelson JM, Layfield LJ, Bean JM, Prosnitz LR, Dewhirst MW, Tumor oxygenation predicts for the likelihood of distant metastases in human soft tissue sarcoma, Cancer Reserach 1996, 56: p. 941-943.

Chaplin DJ, Durand RE, Olive PL, Acute hypoxia in tumors: implications for modifiers of radiation effects, International Journal of Radiation, Oncololgy, Biolpgy, Physics 1986 August; 12(8): p. 1279-1282.

Denko NC, Fontana LA, Hudson KM, Sutphin PD, Raychaudhuri S, Altman R, Giaccia AJ, Investigating hypoxic tumor physiology through gene expression patterns, Oncogene 2003 September 1; 22(37): p. 5907-5914.

Evans SM & Koch CJ, Prognostic significance of tumor oxygenation in humans, Cancer Letters 2003 May 30; 195(1): p. 1-16.

Harris AL, Hypoxia: a key regulatory factor in tumour growth, National Review in Cancer 2002 January; 2(1): p. 38-47.

Kunz M & Ibrahim SM, Molecular responses to hypoxia in tumor cells, Molecular Cancer 2003; 2: p. 23-31.

Rockwell S, Oxygen delivery: implications for the biology and therapy of solid tumors, Oncology Research 1997; 9(6-7): p. 383-390.

Ryan H, Lo J, Johnson RS, The hypoxia inducible factor-1 gene is required for embryogenesis and solid tumor formation, EMBO Journal 1998, 17: p. 3005-3015.

Ryan HE, Poloni M, McNulty W, Elson D, Gassmann M, Arbeit JM, Johnson RS, Hypoxia-inducible factor-1 is a positive factor in solid tumor growth, Cancer Res, August 1, 2000; 60(15): p. 4010 - 4015.