Miskatonic University PressCoursera sends me emails every now and then suggesting courses I might want to try. The emails are filled with stupidly long URLs like this, which I present in 20-character lines:
https://eventing.coursera.org/redirect/
_WtLvKefAb4HnpCdGvrQ
2Bzme2yv6Vbcd4MdVvIA
AHedEK3IwHDRNzGbV5R1
1c43qg-ohDi3F4H4lL1K
qvDPBg.DVNpoJbn4hljT
4VLZbW1Cg.Uz-IOZ2YNh
Fi1RAvvYlFfrp8yzjINi
9_AfhUNf7kYjg2ZfF36n
fY8EiibmNlPznFJVn4ue
8qRs0SM_mWqyoNNQk9Dx
qFeymHudQYCQs2xX3nAT
hQ0lbLN_Rxz1ht8k3aTk
Il3FUWpTfVhkkViMBsvn
y7kkgpArIc9gF1TC8DeU
p9Y8iCfRJfFT5pEkJUF3
VcBWKGUxQ1wZ88i79cCX
xH6WPX11pb-gQgDSSUMN
gK1ZMuZweIsc4tLbjXqS
rUpx8Ot672hFol7a5YSM
5DUBaFhO_5bdEEmIgN9D
J0YzrZuMqDmdzdlZqhVl
UrQbkMHAedLOJPhOXOWm
IMzJZH-KYx-DDys6jsSb
swOemmenthal7dMVIceI
98sB285q1GMrIyZYM2Vq
telTYNMWkperOLU9y7nW
cvg-kNp2cBtiXFV-Lu8z
c_wHdBdHUNd9IS0NdqG1
l0J0CQIIhyCVTlF81agA
B2IrOF0_XPjXNoETLRcv
whOf4OQ-ZUJdHGWUvXiW
sfWqenNfCfFHNanfsaet
vom-h43cK-oVlYMxSk1y
61YKrNZWhGFS4Vll1SO2
jRASohdxl-bEv2dz3YNW
kzlr-PW-KpBYqtUVxe3T
l69PUWmCiPOJ1Aji1zt7
LTCtooooastlt8tBO8gM
xiST4k6qLRxbpkChl6vZ
TWmvTEky58_duy-wibto
3pa_-aVfrdpn2TTEHd73
D76Ageve48W7hS8UG7eP
raJ1EItRnW3K3V5VwMMr
_UU5YVNeuED1Pq9GYXTG
VRbZp071g-iiOP5EE_W2
vKipOa1YnwO2S-LN7Lvc
uBF_nOexEE0daZlKXbjiZi
That is 942 characters long. With lower- and upper-case letters, numbers, underscores and hyphens available, each character in the string can be one of 64 choices, so there are 64^942 possible strings, which Wolfram Alpha says is on the order of 10^1701.
What is going on in Coursera’s notification system?