The water surface of Lake Superior is at an elevation of $180 \mathrm{~m}$ above sea level. The lowest point of the lake floor is $220 \mathrm{~m}$ below sea level. What is the actual depth of the lake at this point?
(A) $40 \mathrm{~m}$
(B) $300 \mathrm{~m}$
(C) $380 \mathrm{~m}$
(D) $400 \mathrm{~m}$
(E) $500 \mathrm{~m}$
The water surface has an elevation of $+180 \mathrm{~m}$, and the lowest point of the lake floor has an elevation of $-220 \mathrm{~m}$. Therefore, the actual depth of the lake at this point is $180-(-220)=400 \mathrm{~m}$.

Answer: (D)