This strange object was built on the coast of Great Britain. What is it?

sun clock
storm detector
acoustic mirror
emergency radio receiver
Prior to World War II and the in­ven­tion of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of de­tect­ing airborne in­va­sion­s. The de­vel­op­ment of radar put an end to further ex­per­i­men­ta­tion with the tech­nique.

Answers distribution

sun clock
storm detector
acoustic mirror
emergency radio receiver
answers
flagFinland
0%
0%
98%
1%
61
flagItaly
0%
4%
94%
1%
74
flagSwitzerland
7%
0%
92%
0%
40
flagthe United Arab Emirates
5%
1%
90%
1%
51
flagGreece
8%
0%
83%
8%
24
flagthe Netherlands
8%
7%
79%
4%
94
flagCzech Republic
10%
10%
78%
0%
19
flagSpain
7%
7%
77%
7%
27
flagGermany
9%
5%
75%
9%
259
flagSweden
17%
3%
75%
3%
29
flagPoland
9%
6%
73%
10%
17171
flagthe United Kingdom
13%
6%
69%
9%
641
flagNorway
15%
6%
68%
9%
32
flagAustria
9%
13%
68%
9%
22
flagthe United States
13%
5%
67%
11%
302
flagRepublic of Ireland
13%
11%
66%
7%
51
flagLithuania
0%
11%
66%
22%
18
flagUnknown
11%
8%
64%
15%
1063
flagFrance
20%
0%
60%
20%
20
flagIndia
30%
18%
25%
22%
70

Gender

men73%
women78%
sun clock
8%
8%
storm detector
6%
5%
acoustic mirror
73%
78%
emergency radio receiver
11%
7%

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This strange object was built on the coast of Great Britain. What is it?
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