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%
100%
0%
47
flagItaly
0%
2%
97%
0%
48
flagVietnam
0%
0%
91%
8%
12
flagSpain
0%
9%
85%
4%
21
flagGreece
7%
0%
85%
7%
14
flagGermany
8%
3%
76%
10%
159
flagthe United Arab Emirates
17%
0%
76%
5%
17
flagthe Netherlands
14%
7%
75%
3%
28
flagPoland
10%
6%
72%
9%
9438
flagNorway
20%
5%
70%
5%
20
flagthe United Kingdom
17%
6%
65%
9%
392
flagthe United States
14%
6%
63%
14%
191
flagCanada
7%
0%
61%
30%
13
flagRepublic of Ireland
20%
6%
60%
13%
15
flagUnknown
15%
0%
59%
21%
32
flagLithuania
0%
14%
57%
28%
14
flagFrance
22%
0%
55%
22%
18
flagIndia
27%
18%
27%
25%
58

Gender

men78%
women75%
sun clock
6%
10%
storm detector
5%
5%
acoustic mirror
78%
75%
emergency radio receiver
9%
9%

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