As illustrated in the diagram to th

As illustrated in the diagram to the right, the Fukushima I nuclear plant has a number of trenches and pipe tunnels that stretch from each unit's reactor (diagram #1), to the unit's turbine building (diagram #2), to the sea (to the right of diagram #6).[57] In some locations these connections are open trenches, while in other locations the connections are pipe tunnels.[57]
During work to restore power to Unit 2 on 27 March, TEPCO reported very high levels of radiation in water in the basement of the Unit 2 turbine building.[58] While first reported radiation levels of more than 10 million times usual appeared later to be erroneous, the radiation measurements were more than 100,000 times higher than usual.[59] On 28 March, the Nuclear Safety Commission announced its suspicion that "radioactive substances from temporarily melted fuel rods at the No. 2 reactor had made their way into water in the reactor containment vessel and then leaked out through an unknown route".[60] Highly radioactive water was later found in trenches at three of the units.[61] These trenches stretch toward, but do not directly connect to, the sea (see diagram #6).[61] On 30 March, the Units 2 and 3 trenches were 1 m below the level at which they would overflow into the sea.[61] In comparison, the Unit 1 trench was 10 cm from overflowing.[61]
The high levels of water in the trenches combined with their potential to overflow to the sea complicated the cooling efforts because the water required to cool the reactor was believed to also be filling the trenches.[62] Hence, cooling Unit 2 with large quantities of fresh water was expected to cause the trenches, leading to the sea, to fill and overflow—worsening the radioactivity release.[62] Consequently, TEPCO reduced the amount of water injected into Unit 2 from 16 to 7 ton per hour.[60] TEPCO used two approaches to prevent the highly radioactive water from leaking into the sea.[63]