Editing Challenger Deep (section)

===1975–1980 – RV ''Thomas Washington''===
The Scripps Institution of Oceanography deployed the 1,490-ton Navy-owned, civilian-crewed research vessel ''[[USNS Thomas Washington (T-AGOR-10)|Thomas Washington]]'' (AGOR-10) to the Mariana Trench on several expeditions from 1975 to 1986. The first of these was the ''Eurydice Expedition, Leg 8'' which brought Fisher back to the Challenger Deep's western basin from 28–31 March 1975.<ref>"Eurydice Expedition, Leg 8, R/V ''Thomas Washington'', Informal Report 24–31 March 1975", [[Scripps Institution of Oceanography|SIO]] Geological Data Center, 27 June 1975.</ref> ''Thomas Washington'' established geodetic positioning by ([[Satellite navigation|SATNAV]]) with Autolog Gyro and EM Log. [[Bathymetry|Bathymetrics]] were by a 12&nbsp;kHz Precision Depth Recorder (PDR) with a single 60° beam. They mapped one, "possibly two", axial basins with a depth of {{convert|10915|±|20|m|ft|0|abbr=on}}.<ref>"Eurydice Expedition, Leg 8, R/V ''Thomas Washington'', Informal Report 24–31 March 1975", [[Scripps Institution of Oceanography|SIO]] Geological Data Center, 27 June 1975, Track Plot, p. 4</ref><ref>{{cite web |url=http://maps.continentalshelf.org/geodas/ERDC08WT.htm |publisher=Continental Shelf Programme |title=Survey Identifier: ERDC08WT |access-date=11 November 2019 |archive-date=15 May 2021 |archive-url=https://web.archive.org/web/20210515055908/http://maps.continentalshelf.org/geodas/ERDC08WT.htm |url-status=live }}</ref> Five dredges were hauled 27–31 March, all into or slightly north of the deepest depths of the western basin. Fisher noted that this survey of the Challenger Deep (western basin) had "provided nothing to support and much to refute recent claims of depths there greater than {{convert|10915|±|20|m|ft|0|abbr=on}}."<ref>Fisher, R., [http://siox.sdsc.edu/secureGetObj.php?filename=SIOGDC_ERDC08WT_20070608200108001_20070608200108001_ERDC08WT_weekly_reports.txt&col=ERDC08WT "Eurydice Expedition Leg 08, Weekly Reports"] {{Webarchive|url=https://web.archive.org/web/20211224220500/http://siox.sdsc.edu/secureGetObj.php?filename=SIOGDC_ERDC08WT_20070608200108001_20070608200108001_ERDC08WT_weekly_reports.txt&col=ERDC08WT |date=24 December 2021 }}, Scripps Institution of Oceanography, 1975.</ref> While Fisher missed the eastern basin of the Challenger Deep (for the third time), he did report a deep depression about 150 nautical miles east of the western basin. The 25 March dredge haul at {{Coord|12|03.72|N|142|33.42|E}} encountered {{convert|10015|m|ft}}, which pre-shadowed by 22 years the discovery of HMRG Deep/[[Sirena Deep]] in 1997.<ref>{{cite AV media |title=ERDC08WT-024D |url=https://library.ucsd.edu/dc/object/bb4745506c |author=Thomas Washington |type=Dredge |location=Scripps Institution of Oceanography, Geological Collections |date=1975 |id=IGSN: SIO001559 |access-date=11 April 2022 |archive-date=17 October 2015 |archive-url=https://web.archive.org/web/20151017090236/http://library.ucsd.edu/dc/object/bb4745506c |url-status=live }}</ref> The deepest waters of the HMRG Deep/Sirena Deep at {{convert|10714|±|20|m|ft|0|abbr=on}} are centered at/near {{Coord|12|03.94|N|142|34.866|E}}, approximately 2.65&nbsp;km from Fisher's 25 March 1975 {{convert|10015|m|ft}} dredge haul.

On Scripps Institution of Oceanography's ''INDOPAC Expedition Leg 3'',<ref>{{cite web |url=http://maps.continentalshelf.org/geodas/INDP03WT.htm |publisher=Continental Shelf Programme |title=Survey Identifier: INDP03WT |access-date=21 January 2020 |archive-date=28 September 2020 |archive-url=https://web.archive.org/web/20200928043559/http://maps.continentalshelf.org/geodas/INDP03WT.htm |url-status=live }}</ref> the chief scientist, Dr. Joseph L. Reid, and oceanographer Arnold W. Mantyla made a hydrocast of a free vehicle<ref>INDOPAC Expedition, Leg 3, R/V. Thomas Washington Informal Report, SIO INDP03WT)</ref> (a special-purpose [[benthic lander]] (or "baited camera") for measurements of water temperature and salinity) on 27 May 1976 into the western basin of the Challenger Deep, "Station 21", at {{Coord|11|19.9|N|142|10.8|E}} at about {{convert|10840|m|ft}} depth.<ref>Indopac Expedition Leg 3 Weekly Summary J. Reid</ref><ref>Taira, K, "Deep CTD Casts in the Challenger Deep", ''Journal of Oceanography'', Vol. 61, 2005, p. 450</ref> On ''INDOPAC Expedition Leg 9'', under chief scientist A. Aristides Yayanos, ''Thomas Washington'' spent nine days from 13–21 January 1977 conducting an extensive and detailed investigation of the Challenger Deep, mainly with biological objectives.<ref>INDOPAC Expedition, Leg 9, R/V. Thomas Washington Informal Report, SIO INDP09WT</ref> "Echo soundings were carried out primarily with a 3.5 kHz single-beam system, with a 12 kHz echosounder operated in addition some of the time" (the 12&nbsp;kHz system was activated for testing on 16 January).<ref>Cruise Report, INDOPAC Expedition, Leg 9</ref> A benthic lander was put into the western basin ({{Coord|11|19.7|N|142|09.3|E}}) on 13 January, bottoming at {{convert|10663|m|ft}} and recovered 50 hours later in damaged condition. Quickly repaired, it was again put down on the 15th to {{convert|10559|m|ft}} depth at {{Coord|11|23.3|N|142|13.8|E}}. It was recovered on the 17th with excellent photography of [[amphipods]] (shrimp) from the Challenger Deep's western basin. The benthic lander was put down for the third and last time on the 17th, at {{Coord|11|20.1|N|142|25.2|E}}, in the central basin at a depth of {{convert|10285|m|ft}}. The benthic lander was not recovered and may remain on the bottom in the vicinity of {{Coord|11|20.1|N|142|25.2|E}}. Free traps and pressure-retaining traps were put down at eight locations from 13 to 19 January into the western basin, at depths ranging from {{convert|7353|to(-)|10715|m|ft}}. Both the free traps and the pressure-retaining traps brought up good sample amphipods for study. While the ship briefly visited the area of the eastern basin, the expedition did not recognize it as potentially the deepest of the three Challenger Deep basins.<ref>{{cite web |url=https://gis.ngdc.noaa.gov/viewers/redirect.html?layers=multibeam&minx=144.00209&maxx=155.49962&miny=-41.348255&maxy=13.947198 |title=Bathymetric Data Viewer |publisher=[[NOAA]] |access-date=11 November 2019 |archive-date=24 September 2020 |archive-url=https://web.archive.org/web/20200924011537/https://maps.ngdc.noaa.gov/viewers/bathymetry/?layers=multibeam&minx=144.00209&maxx=155.49962&miny=-41.348255&maxy=13.947198 |url-status=live }}</ref>
	
''Thomas Washington'' returned briefly to the Challenger Deep on 17–19 October 1978 during ''Mariana Expedition Leg 5'' under chief scientist James W. Hawkins.<ref>{{Cite book |chapter-url=https://library.ucsd.edu/dc/object/bb4202429z |title=MARA05WT Mariana Leg 5 – Cruise Report |publisher=Geological Data Center at Scripps Institution of Oceanography |date=January 1979 |doi=10.6075/J01J981F |author1=Geological Data Center at Scripps Institution of Oceanography |chapter=Cruise Report |access-date=11 November 2019 |archive-date=11 November 2019 |archive-url=https://web.archive.org/web/20191111075423/https://library.ucsd.edu/dc/object/bb4202429z |url-status=live }}</ref> The ship tracked to the south and west of the eastern basin, and recorded depths between {{convert|5093|and(-)|7182|m|ft}}. Another miss. On ''Mariana Expedition Leg 8'', under chief scientist Yayanos, ''Thomas Washington'' was again involved, from 12–21 December 1978, with an intensive biological study of the western and central basins of the Challenger Deep.<ref>{{cite web |url=http://maps.continentalshelf.org/geodas/MARA08WT.htm |publisher=Continental Shelf Programme |title=Survey Identifier: MARA08WT |access-date=11 November 2019 |archive-date=28 September 2020 |archive-url=https://web.archive.org/web/20200928063930/http://maps.continentalshelf.org/geodas/MARA08WT.htm |url-status=live }}</ref> Fourteen traps and pressure-retaining traps were put down to depths ranging from {{convert|10455|to(-)|10927|m|ft}}; the greatest depth was at {{Coord|11|20.0|N|142|11.8|E}}. All of the 10,900-plus m recordings were in the western basin. The {{convert|10455|m|ft}} depth was furthest east at 142°26.4' E (in the central basin), about 17&nbsp;km west of the eastern basin. Again, focused efforts on the known areas of extreme depths (the western and central basins) were so tight that the eastern basin again was missed by this expedition.<ref>Yayanos, A. A. et.al, "Dependence of Reproduction Rate on Pressure as a Hallmark of Deep-Sea Bacteria", ''Applied and Environmental Microbiology'', December 1982, pp. 1356–1361</ref>

From 20 to 30 November 1980, ''Thomas Washington'' was on site at the western basin of the Challenger Deep, as part of ''Rama Expedition Leg 7'', again with chief-scientist Dr. A. A. Yayanos.<ref>{{cite web |url=http://maps.continentalshelf.org/geodas/RAMA07WT.htm |publisher=Continental Shelf Programme |title=Survey Identifier: RAMA07WT |access-date=11 November 2019 |archive-date=28 September 2020 |archive-url=https://web.archive.org/web/20200928050205/http://maps.continentalshelf.org/geodas/RAMA07WT.htm |url-status=live }}</ref> Yayanos directed ''Thomas Washington'' in arguably the most extensive and wide-ranging of all single-beam bathymetric examinations of the Challenger Deep ever undertaken, with dozens of transits of the western basin, and ranging far into the [[backarc]] of the Challenger Deep (northward), with significant excursions into the Pacific Plate (southward) and along the trench axis to the east.<ref>{{cite web |url=https://maps.ngdc.noaa.gov/viewers/bathymetry/ |title=Bathymetric Data Viewer |publisher=[[NOAA]] |access-date=9 November 2019 |archive-date=9 July 2017 |archive-url=https://web.archive.org/web/20170709195021/https://maps.ngdc.noaa.gov/viewers/bathymetry/ |url-status=dead }} [select single beam and enter RAMA07WT]</ref> They hauled eight dredges in the western basin to depths ranging from {{convert|10015|to(-)|10900|m|ft}}, and between hauls, cast thirteen free vertical traps. The dredging and traps were for biological investigation of the bottom. In the first successful retrieval of a live animal from the Challenger Deep, on 21 November 1980 in the western basin at {{Coord|11|18.7|N|142|11.6|E}}, Yayanos recovered a live amphipod from about 10,900 meters depth with a pressurized trap.<ref>Yayanos, A. A., [https://www.globaloceandesign.com/uploads/3/0/7/4/30747513/2009_into_the_trench_part_2.pdf "Recovery of Live Amphipods at over 102MPa from the Mariana Trench"] {{Webarchive|url=https://web.archive.org/web/20200922225246/https://www.globaloceandesign.com/uploads/3/0/7/4/30747513/2009_into_the_trench_part_2.pdf |date=22 September 2020 }}, ''Marine Technology Society Journal'', Winter 2009, Volume 43, No. 5, p. 134.</ref> Once again, other than a brief look into the eastern basin, all bathymetric and biological investigations were into the western basin.<ref>Yayanos, F., Informal Report and Index of Navigation, Depth and Magnetic Data – Rama Expedition, Leg 7, 31 October 1980 – 1 December 1980, R/V Thomas Washington (GDC Cruise I.D. #181)</ref>