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Two floors of bubbler pools beneath the reactor served as a large water reservoir for the emergency cooling pumps and as a pressure suppression system capable of condensing steam in case of a small broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released by a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. After the disaster, the pools and the basement were flooded because of ruptured cooling water pipes and accumulated firefighting water.{{citation needed|date=April 2022}}
The smoldering graphite, fuel and other material above, at more than {{convert|1200|C|F}},<ref name=lava2>{{cite journal |doi=10.1134/S1087659609020126 |title=Behavior of melts in the UO2-SiO2 system in the liquid-liquid phase separation region |year=2009 |last1=Petrov |first1=Yu. B. |last2=Udalov |first2=Yu. P. |last3=Subrt |first3=J. |last4=Bakardjieva |first4=S. |last5=Sazavsky |first5=P. |last6=Kiselova |first6=M. |last7=Selucky |first7=P. |last8=Bezdicka |first8=P. |last9=Jorneau |first9=C. |last10=Piluso |first10=P. |journal=Glass Physics and Chemistry |volume=35 |issue=2 |pages=199–204|s2cid=135616447 }}</ref> started to burn through the reactor floor and mixed with molten concrete from the reactor lining, creating [[corium (nuclear reactor)|corium]], a radioactive semi-liquid material comparable to [[lava]].<ref name=Lava1>{{cite journal |doi=10.1134/S1066362208050131 |title=Formation and spread of Chernobyl lavas |year=2009 |last1=Bogatov |first1=S. A. |last2=Borovoi |first2=A. A. |last3=Lagunenko |first3=A. S. |last4=Pazukhin |first4=E. M. |last5=Strizhov |first5=V. F. |last6=Khvoshchinskii |first6=V. A. |journal=Radiochemistry |volume=50 |issue=6 |pages=650–654|s2cid=95752280 }}</ref><ref name=lava3>{{Cite news |last1=Journeau |first1=Christophe |last2=Boccaccio|first2=Eric|last3=Jégou|first3=Claude |last4=Piluso|first4=Pascal |last5=Cognet|first5=Gérard |title=Flow and Solidification of Corium in the VULCANO Facility |publisher=Commissariat à l'énergie atomique et aux énergies alternatives |series=Engineering case studies online|year=2001 |oclc=884784975|citeseerx=10.1.1.689.108 }}</ref> It was feared that if this mixture melted through the floor into the pool of water, the resulting steam production would further contaminate the area or even cause a steam explosion, ejecting more radioactive material from the reactor. It became necessary to drain the pool.<ref>{{cite book |last=Medvedev |first=Z. |year=1990 |pages=[https://archive.org/details/legacyofchernoby00medv/page/58 58–59] |title=The Legacy of Chernobyl |url=https://archive.org/details/legacyofchernoby00medv |url-access=registration |publisher=W W Norton & Co Inc |isbn=978-0-393-30814-3}}</ref> These fears ultimately proved unfounded, since corium began dripping harmlessly into the flooded bubbler pools before the water could be removed.<ref>{{cite journal |last1=Checherov |first1=Konstantin |title=The Unpeaceful Atom of Chernobyl |journal=Person |date=2006 |issue=1}}</ref> The molten fuel hit the water and cooled into a light-brown ceramic pumice, whose low density allowed the substance to float on the water's surface.<ref>{{
Unaware of this fact, the government commission directed that the bubbler pools be drained by opening its [[sluice gates]]. The valves controlling it, however, were located in a flooded corridor in a subterranean annex adjacent to the reactor building. Volunteers in [[diving suit]]s and [[respirator]]s (for protection against radioactive [[aerosol]]s), and equipped with [[dosimeter]]s, entered the knee-deep radioactive water and managed to open the valves.<ref name="kramer">{{cite web |last1=Kramer |date=26 April 2016 |first1=Sarah |title=The amazing true story behind the Chernobyl 'suicide squad' that helped save Europe |url=http://www.businessinsider.com/chernobyl-volunteers-divers-nuclear-mission-2016-4 |website=[[Business Insider]] |access-date=7 October 2016 |archive-url=https://web.archive.org/web/20161009180156/http://www.businessinsider.com/chernobyl-volunteers-divers-nuclear-mission-2016-4 |archive-date=9 October 2016 |url-status=live }}</ref><ref name="mkru">{{cite web |last1=Samodelova |first1=Svetlana |script-title=ru:Белые пятна Чернобыля |url=http://www.mk.ru/politics/sng/2011/04/25/584047-belyie-pyatna-chernobyilya.html |website=Московский комсомолец |access-date=7 October 2016 |date=25 April 2011 |language=ru |archive-url=https://web.archive.org/web/20161009160659/http://www.mk.ru/politics/sng/2011/04/25/584047-belyie-pyatna-chernobyilya.html |archive-date=9 October 2016 |url-status=live }}</ref> These were the engineers [[Alexei Ananenko]] and [[Valeri Bezpalov]] (who knew where the valves were), accompanied by the shift supervisor [[Boris Baranov]].<ref name=divers2>{{cite news |date=15 May 1986 |title=Soviets Report Heroic Acts at Chernobyl Reactor With AM Chernobyl Nuclear Bjt |website=[[Associated Press]] |url=https://apnews.com/bfb4a0cf2479ee940116c74141e8a332 |access-date=26 April 2014 |archive-url=https://web.archive.org/web/20140429204527/http://www.apnewsarchive.com/1986/Soviet-Press-Reports-Heroic-Acts-at-Chernobyl-Reactor-With-AM-Chernobyl-Nuclear-Bjt/id-bfb4a0cf2479ee940116c74141e8a332 |archive-date=29 April 2014 |url-status=live }}</ref><ref>{{cite web |url=http://www.myslenedrevo.com.ua/uk/Sci/HistSources/Chornobyl/1986/05/16/ChernobylAdresMuzhestva.html |script-title=ru:Чернобыль: адрес мужества |trans-title=Chernobyl: the address of courage |last1=Zhukovsky |first1=Vladimir |last2=Itkin |first2=Vladimir |last3=Chernenko |first3=Lev |date=16 May 1986 |website=[[Telegraph Agency of the Soviet Union|TASS]] |language=ru |access-date=5 November 2018 |archive-url=https://web.archive.org/web/20181108224502/https://www.myslenedrevo.com.ua/uk/Sci/HistSources/Chornobyl/1986/05/16/ChernobylAdresMuzhestva.html |archive-date=8 November 2018 |url-status=live }}</ref><ref name="Chernobyl 1986, p.178">{{cite book |title=Chernobyl: The End of the Nuclear Dream |date=1986 |page=178 |first=Nigel |last=Hawkes |display-authors=etal |location=London |publisher=Pan Books |isbn=978-0-330-29743-1}}</ref> All three men were awarded the [[Order For Courage]] by [[President of Ukraine|Ukrainian President]] [[Petro Poroshenko]] in May 2018.<ref>{{cite web | url=https://www.ukrinform.ru/rubric-society/2449795-prezident-vrucil-nagrady-geroamlikvidatoram-i-rabotnikam-caes.html | script-title=ru:Президент Петр Порошенко вручил государственные награды работникам Чернобыльской атомной электростанции и ликвидаторам последствий аварии на ЧАЭС. | trans-title=President Petro Poroshenko presented state awards to employees of the Chernobyl nuclear power plant and the liquidators of the consequences of the Chernobyl NPP accident. | language=ru | access-date=28 May 2019 | archive-url=https://web.archive.org/web/20190514115713/https://www.ukrinform.ru/rubric-society/2449795-prezident-vrucil-nagrady-geroamlikvidatoram-i-rabotnikam-caes.html | archive-date=14 May 2019 | url-status=live }}</ref>
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