content: Add ability to place exhibits underground. #193

Job	Run time
no_std	47s
miri	1m 58s
build (ubuntu, stable, locked, true)	59m 48s
build (windows, stable, locked, false)	27m 34s
build (macos, stable, locked, false)	19m 12s
build (ubuntu, nightly, locked, false)	34m 17s
build (ubuntu, beta, locked, false)	37m 22s
build (ubuntu, stable, latest, false)	17m 59s
build (ubuntu, stable, minimal, false)	33m 34s
fuzz	18m 24s
deploy	5s
	4h 11m 0s

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